It's something you'll see mentioned around here - V4V - Value for value. Primarily I use it to send someone a few Sats when I really appreciate their post. You can think of them as tips. There's also a regular 'like' feature, but I save my zaps for when someone contributes something I find useful. And really, zaps are just a fraction of a penny. I load up my wallet with maybe 1000 sats (less than $1) and between the zaps I send and the ones I receive, it takes a very long time to go through that much.