This is sort of like cash... You can buy stuffs from the Store. You can buy lottery ticket, username glow, color, monthly arcade pass etc.. from there. Speaking of arcade pass, you can also play various games in the Arcade. Each time you play a game, it costs you 10 goop. But if you have the monthly pass, you can play as much as you want in a month for 500 goop.

There are also other uses of goop, like awarding someone for making you a signature picture or any other favours.

Hope that helps