> but I don't fully understand how rebase works yet

Git maintains a tree of commits, each commit pointing to its parent. When you rebase, by default git finds the last-common-ancestor commit of the branch you're rebasing and the branch you're rebasing onto. That common ancestor is the base, and the subsequent commits on the branch you're rebasing are replayed on the latest commit of the branch you're rebasing onto.

> Or the hard flag

You mean reset --hard?