Jokes aside, there is a wealth of literature on how to understand git from the ground up. I strongly recommend reading through one of those. I can't recommend anything in particular, but I'm sure someone else will chime in with their favourite.
Since you asked, I just published Head First Git. I did a "Show HN" that didn't get much traction, but rather than repeating what I said there, I'll just post the link—https://news.ycombinator.com/item?id=30072348
[end shameless plug]
And to the point many others made, there are a _lot_ of resources out there, but if you are looking to start back at the basics, I feel my book (which is designed for beginners) might be a good start.
Of course there is the canonical Pro Git (https://git-scm.com/book/en/v2) book.
And once you understand the _Directed Acyclic Graph_ I feel https://github.com/git-school/visualizing-git is a great project to see what happens when you run certain operations in Git.
Perhaps you've concluded that I love Git, and I love teaching it—I am @looselytyped on Twitter (DMs are open) if you want to reach out and we can carry out a conversation elsewhere—always willing to talk Git.
Edit—I replied to the wrong comment. My apologies kqr. This was meant for VeninVidiaVicii.