Zap is a term that is synonymous to Likes, Favorite etc in other platforms except that it sends one a fraction of Bitcoin. So instead of me just saying - hey I like your post, I actually send a value that u can do something with. 

Here’s an article that might help https://thebitcoinmanual.com/articles/nostr-zaps/

If you do want to set this up you can get started with @Wallet of Satoshi