When one cracks a C, they are opening a canned soda. It refers to the "crack" sound made by the can. Pretty funny. C stands for Cola too, incase you're wondering.
Joey: Im tired, better crack a C
Kyle: OMG, this kid just cracked a C at 5:00 A.M.
Will: OMG, 5 AM C's are a bad idea.
Joey: Josh cracks C's all the time, he doesn't sleep.