It is a French tradition, which occurs just before the dinner.
It is when you drink couple of drinks (especially alcoholic stuffs) and you eat some chips, peanuts, almonds, ...
The goal is to prepare your stomach to eat, and to have fun of course!!
Emmanuel : Hey Eric do you want to come over for dinner?
Eric : Of course dude! Do you want me to bring something for the predinning?
Emmanuel : Yeah bring some beers and chips
Eric : OK! See you!
Eric : Hey Manu, I am not very hungry right now, don't you want to have a quick predinning before eating?
Emmanuel : Of course man! I'll grab some beers and peanuts