It's a monorepo, where some of the contents are a copy of another existing repo.

I'm sure we've all 'vendored in' a dependency before - just copying in the files and checking them in. The fiddly bit is when you then want to update that vendored dependency to the newer version (more manual copying of files and checking in), and it gets tedious and error prone when you have 10 of them. Subtrees do the same thing, but provide tooling to make it a lot easier.

Yes - or rather, it's a tool that helps you build a monorepo from separate repos.