Simpler explanation of path for GIT checkout command

Question

In the git documentation for the checkout command the following text is found:

...If no paths are given, git checkout will also update HEAD to set the specified branch as the current branch....

Can anyone give a simpler explanation of what this means? I'm sorry if it seems simple, and reading through that page, I can't seem to come up with what it means exactly. Sorry if this seems basic..

In particular I am confused on how checkout is updating HEAD. I usually envision checkout affecting the working directory -- is this an ability unique to git in that you are updating your local copy of the repository for the purposes of working with it later?

Answer 1

Version A: (specifying only the branch)

git checkout <branch>

Gets all files for that <branch> and places HEAD (a pointer to "where am I now") at the branch specified.

Version B: (specifying a path only)

git checkout <file>

Gets the latest version of <file> and leaves HEAD alone.