Avarice is a fancy word for good old-fashioned greed. It's one of what some call "the seven deadly sins."
Do you want more and more money? Or cookies? Or video games? Or anything? Then your heart is full of avarice, which you probably know better as greed. When people talk about greed, it's clearly not a good thing, but avarice has an even worse flavor to it. Avarice is often looked upon as a sin, and it's always considered despicable and evil.