Sn = tin
This is pretty self-explainatory, if you studied chemistry in high-school. It is a phun on the trademarked brand Snapple, which is a collection of juice and fruit drinks. Cueball gets an offer to try out a Snapple in the form of an apple and tries to bites it, but is not able to. Then, he learns that the offering is actually an apple infused with Tin. The periodical table notation for Tin (so says the image text also) is Sn. Combined with apple, you get Snapple.