It means to buy a lot of a particular thing, in case you cannot get it later, or because it might be more expensive later.

eg: When there's a storm coming we always stock up on food and candles.

stock up的意思是特意大量购买某种东西，以备不时之需或者以免将来价格增高。

例：我们经常储存大量食品和蜡烛以防暴风雨来临