What are the git concepts of HEAD, master, origin?

As I’m learning about git, I keep coming across the terms HEAD, master, origin, and I’m not sure what the differences are. If I understand correctly, HEAD is always equal to the latest revision? And if so, is that the latest revision of the whole repository, or of a specific branch or tag? This is so confusing. I’ve read so many tutorials on this and things like branching/merging, but still can’t wrap my head around it.