No matter how media-savvy or un-media-savvy (hi, mom!) you may be, you're bound to be familiar with the phenomenon. The ship is trying to fire its proton lasers, but some engineer explains to the captain that quantum interference is preventing them from locking on to the enemy ship. Or the robot has a safety inhibitor that means he can't attack humans. Or the quark reactor is overloading and someone needs to go and eject it before it goes critical. This is what we call technobabble, and I think it's an overly maligned tool in the writer's toolbox.
Technobabble, as it is generally understood, is any bit of dialogue which attempts to explain why something is or isn't possible due to the rules of the universe in which the story takes place, and thus prevents the story from ending too quickly. Despite the name, it isn't necessarily a sci-fi thing, although it's almost always found in genre fiction of some kind. (For example, in fantasy: "You can't use wizard magic to undo a dragon's curse!" Or in horror: "The ghost won't be satisfied until we find the descendents of all four people who killed her!") It's more common in genre fiction, I believe, because the author is already creating the rules of their universe—whether they be rules about magic, technology, vampires, or what have you—and thus is freer to bend these rules as necessary. Tom Clancy can't just make up facts about nuclear fission, because it would contradict real-world physics ("If you smother the plutonium in cheese sauce, it doubles the explosive force!"), but Stephanie Meyer is free to come up with whatever nonsense she likes about her vampires, because there's no established reality to contradict.
In its most benign form, technobabble is what I like to think of as story grease: it greases the gears of the story, makes it move more smoothly from point A to point B. It simplifies the process of getting the characters to do what you want in the service of the story—or, at times, preventing them from doing things that would end the story too quickly.
As an example, consider the transporters of Star Trek. They are, first, technobabble that lets characters travel from point A to point B instantaneously, without all the hassle of launching a shuttlecraft. (Also, it's cheaper for the show: rather than having to build a set for the shuttle and put together the special effects sequence of it landing on a foreign world, you can just slap some sparkle effects on your characters, and *poof*, they're there.) However, there's more than one story in Star Trek where a problem arises that could be solved easily if the characters simply transported to another location. In this case, you need a second bit of technobabble to explain why the transporters can't be used in this case—often something along the lines of "particle interference" or "power fluctuations."
I want to point out that I don't think technobabble is necessarily a bad thing. It can be used poorly, just like every other story tool. I think that, because technobabble is almost exclusively a genre tool, and genre fiction is generally less well-accepted than "serious" fiction, it gets a bad rap.
I think technobabble either goes too far or not far enough. (Wow, how specific!) Either it's too detailed and stuffed with unnecessary information that just confuses the reader/viewer/player/etc., or it's not enough to truly explain what's going on. (The site TVTropes uses the phrase "Fridge Logic" for the second case. Take a look at that article; it's pretty interesting.) You get more of the first case in long-running series or franchises, in which every episode is another opportunity to contradict something from a previous episode. (A.K.A. "Why couldn't Kirk just ask the all-powerful energy being from last week's show to help defeat the all-powerful energy being in this week's show?") You get more of the second in one-off movies or shows that only last a season, where they don't have the time to explain every last nuance of their crazy technology/magic/ghosts/etc.
So what makes for good technobabble? Basically, that it allows the story to continue without ever getting in the way. At the end of the episode/book/game, you should remember the emotional high and low points, not why the ship's laser guns couldn't damage the space amoeba. It should be just enough to float the story without disrupting the suspension of disbelief.
One of my favorite examples of technobabble isn't even "techno" at all; in fact, it's decidedly low-tech. In The Mummy (the 1999 one, not the 1932 one), the several-millenia-old titular preserved corpse has just been reawakened and is starting to murder indiscriminately, as is the wont of all movie monsters. He approaches Beni, a scummy little weasel of a man; Beni responds by opening his shirt and pulling out religious symbols from every faith, chanting bits of prayers in the appropriate languages. Naturally, none of them work, but when he pulls out a star of David and says something in Hebrew, the mummy stops in surprise. "The language of the slaves!" he says in (I presume) Hebrew. "I may have some use for you after all..."
Does it make sense for Beni—or the mummy, for that matter—to be fluent in Hebrew? Not really. Even though the mummy later switches to Coptic (the language of ancient Egypt, according to Wikipedia), would Beni know how to speak Coptic? Again, probably not. But this explanation why Beni can communicate with the mummy is just enough to keep the story moving without forcing some lengthy dialogue on the audience. It takes all of two seconds, then boom, Beni can go on to be the mummy's right-hand man.
Another alternative to technobabble is to create your universe with very specific, well-designed, internally consistent rules, and then only tell the audience as much as they need to know. The franchise that exemplifies this is Mass Effect, a video game series whose highly anticipated third installment is launching this Tuesday. The series' technology is incredibly thorough and consistent: not only did they come up with a substance that allows both faster-than-light travel and personal force fields, they also thought out how this would affect space battles and fleet engagements. And yet the player doesn't need to know any of it in order to play the game. There's a "codex" that has hundreds of articles about various aspects of technology, but absolutely none of it is essential reading. You might say this is wasted effort on the part of the creators—if the player doesn't need to know it, why include it? But they've made it clear that the franchise is going to continue for as long as it's profitable, so the more effort they put into making a consistent universe at the start, the less effort it will take to "maintain" it for future installments.