So I see a lot of #trending tags but idk what the use for it is

I don't think it does anything. It makes the project show up if you search “trending”, but I don't know why anyone would do that. I'm pretty sure the actual trending page doesn't care about the “trending” tag.

It’s a misconception. People thinks it makes them trending but really it doesn’t. The only tags that help you get on trending are #games #animations #stories #tutorials #art and #music.