What is this weird green thing that I see in videos on the floor?

This is so vague, lord. I think you’re talking about navmesh

When I was watching a tutorial, I noticed that there was something green on the ground and wanted to know what it was, what it does, and how to use it.

Oh. Thanks!

I’m still new to this

Attaching a screenshot will get you a better answer faster.

you can hit P to enable and disable the visibiltity