Commit
Unlike git, mesa uses the word commit to refer to an atomic change in your repo. Whenever you make any mutation to your repo, mesa creates a new commit for you. Consequently, mesa can avoid handling staged/unstaged changes separately, as well as enable you to checkpoint at any individual state of the repo you (or your agents) have ever made.
Change
Changes are what git refers to by the word commit. In mesa terminology, a change is a collection of nodes within one simple path of the history graph. Each change has a unique hexadecimal ID.
Bookmark
A bookmark is a named label which you can associate with a change. As new commits get added to the change, mesa automatically advances the bookmark to the last commit.