What do git checkouts really mean?

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As you noted, HEAD is a label noting where you are in the commit tree. It moves with you when you move from one commit to another. git checkout <commit> is the basic mechanism for moving around in the commit tree, moving your focus (HEAD) to the specified commit.

The commit can be specified by any of a number of ways, commit hash, branch name, tag name, the relative syntax (HEAD^, HEAD~1, etc.) and so on. It is often useful to consider a checkout to be changing branches, and there are some options that work from that perspective, but they all reference commits.

To checkout a commit has some side effects other than moving HEAD around.

  • The working directory is updated to the state of the checked out commit.
  • if a branch name is specified, checkout makes that branch active. The active branch will move along with any new commits that are added.
    • with the -b option a new branch will be created based on the current commit and then made active.
    • with the --track option the checked out branch can be made aware of a remote branch
    • with the --orphan option a new branch is created (like with -b) but will not be based on any existing commit.

There are a few more options, which you can read about in the git checkout man-page, all of which revolve around moving from one commit to another — just varying in what effect that move has in addition to moving HEAD.

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