What does “Changes not staged for commit” mean

when you change a file which is already in the repository, you have to git add it again if you want it to be staged.

This allows you to commit only a subset of the changes you made since the last commit. For example, let’s say you have file a, file b and file c. You modify file a and file b but the changes are very different in nature and you don’t want all of them to be in one single commit. You issue

git add a
git commit a -m "bugfix, in a"
git add b
git commit b -m "new feature, in b"

As a side note, if you want to commit everything you can just type

git commit -a

Hope it helps.