I was making my dinner, and i looked at the bag of snap peas i took from the freezer. It says i

I was making my dinner, and i looked at the bag of snap peas i took from the freezer.  It says in clear print that there are 40 calories per serving and each serving has like 6 peas.  Why the hell do peas have calories???  And since I’m on the subject, what the heck ARE calories and how are they calculated?  I don’t recall ever learning about this in science class but then again I never really paid much attention in science OR math.