What do git checkouts really mean?

Question

What are checkouts in git?

I know once you do checkout to a particular branch, the HEAD points to that branch. But what does

Answer 1

"To check out" means that you take any given commit from the repository and re-create the state of the associated file and directory tree in the working directory.

When you check out a commit that is not a branch head (e.g. git checkout HEAD~2), you are on a so-called detached head. You can create commits here, but once you switch to a different branch, those commits will not be recoverable by a branch name and might even get removed by the garbage collector after some time.