I have several items that give me a plus on crit chan (I assume that is short for critical channel) but what is that? What does it give me?

Critical chance. It's a game mechanic that represents your character's chance to sucessfully score a critical hit (non-sneak attack critical, those are always crits) on an enemy. It works as a perecentage and the game literally "rolls the dice" and generates a random number every time you attack. If your critical chance with weapon A and perk B is 15%, then if the random number generated is 15 or below, the game awards you a critical hit and the extra damage and effects that come with it.

-Gunny