Squick is basically another term for "Eeeewwwww, gross!" A negative emotional response, more specifically a disturbed or disgusted one.
Not to be confused with squee, the very opposite. We hope.
An important point about squick, however, is that something that squicks you may not squick someone else.
How Do You Know It's Squick?Edit
If something makes your stomach heave, or makes you want to puke, or makes you want to stab your eyeballs out so you can never see it again, it's probably squick.
Squick commonly is some sort of lemon, but not always.