k so what does it mean to become a fan of an item

Well, when you're a fan of a content, that means you like that content.

In another way, that shows you appreciate the effort that the Fanpopper, who posted that content, has given. It's like a virtual 'pat on the back' for the contributor of that content. If someone 'fans' a content that I posted, I'll feel happy, you know what I mean? :)