The term "crack!fic" covers a wide range of fanfiction stories with ridiculously implausible premises. The exclamation point, in fanfiction lingo, denotes a modifier; for example, a Harry Potter story featuring "angry!Harry" focuses on the angry side of Harry's character. The modifier in "crack!fic" (also called "crackfic" or just "crack") has two meanings: first, that readers might find these crazy stories addictive, just like crack; and second, that the writer must have been on crack in order to produce them.
The origin of crack!fic is usually much more harmless than the name would suggest. Fanfiction writers often get story ideas, known as "plot bunnies," which they can't get out of their heads. Not all of these ideas are good or even plausible, but sometimes they beg to be written down anyway. Crack!fic ideas can range from silly to funny to disturbing; many are enjoyable on their own merits, but some merely exist as a way for writers to get ridiculous ideas out of their system so they can write something more serious.
Unlikely romantic pairings are one source for crack!fic. These are especially common in the Harry Potter fandom, where pairings like Hogwarts/Giant Squid and Ron/Scabbers abound. What distinguishes a crack pairing from an ordinary ship is not only the unlikeliness but how the author sets it up. In the hands of a good writer, even an unusual pairing can become plausible; if the fanfic is labeled as crack, however, this prompts the readers to accept the lack of a realistic explanation and appreciate the pairing for its craziness.
Crossovers also tend to support crack!fic, if the crossover material is ridiculous and unlikely. In some fandoms, particularly Stargate Atlantis, Alternate Universe (AU) stories tend to turn into crack!fic; for example, the AU where the crew of Atlantis are all houseplants. Sometimes entire fandoms pop up based on crack!fic-there's no other way to explain the existence of fanfiction stories for games like Tetris, Free Cell and Pong.
So is crack!fic good? It depends. Many fanfic writers (especially younger ones) produce crack!fic that purports to be funny, but is really just a collection of inside jokes that only make sense to the writer and a few friends. Many crack!fics are disappointing, in that they take a premise that is not inherently crack, but something that could be plausible and a legitimately good story in the right hands, and lessen it. When done well, however, crack!fic can be humorous and fun to write and read.