I'll be honest, as a fairly skilled and experienced programmer who isn't a git expert, I know what HEAD means, but when I'm rebasing I really have no idea. It all seems to work out in the end because my collaborative work is simple and usually 2–3 people only, so I'm never rebasing against a ton of commits I lack context for (because 90% of them are my commits since I'm usually dealing with PRs to my open source projects rather than someone else's).
HEAD is "the thing we're editing now" but that's not terribly useful when rebasing since you're repeatedly editing a fake history.
