What does "Changes not staged for commit" mean

I thought if you want to track the files you should git add [files you want to track]

I don't know why I got the messages Changes not staged for commit.

If those files were not staged, shouldn't git shows me those files were Untracked like that

All I've done was create a new feature from develop branch and worked in feature/change_excel_format branch

I thought Those files should be in staged status,

But git status told me Changes not staged for commit

To brief, I only know there are 3 stages in git untracked, staged, committed Can any one tell me , what was the stage in for Changes not staged for commit

So if I modified the file a (already in the repo)

and type git st , the git will tell me Changes not staged for commit

if I git a then the file a will be in staged status

if I modified the file a now, there will be two status of file a in git, right ?

So I have to decide if make the staged a be commit or make the not stage a to be staged, and then the previous staged file awill be discard ?

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