I found this on Wikipedia:

Using a system of upvotes and downvotes users can influence what content appears at the top of the main feeds and of each community.

So if you find a post interesting, you can upvote it. And if someone posts cat pictures in asklemmy, you can downvote it, because it's off-topic and maybe you want to discourage such behaviour.

I'm not sure if Lemmy has upvote counters for users, but either way if you upvote a post or comment you say that it's useful/interesting, and it's a bit like a "thank you" in real life.