[git] What does "Changes not staged for commit" mean

It's another way of Git telling you:

Hey, I see you made some changes, but keep in mind that when you write pages to my history, those changes won't be in these pages.

Changes to files are not staged if you do not explicitly git add them (and this makes sense).

So when you git commit, those changes won't be added since they are not staged. If you want to commit them, you have to stage them first (ie. git add).