What does Changes not staged for commit mean

Question

As far as I know, to track a file you have to use: git add [files you want to track]. But I get this message: Changes not staged for commit. IF this is the case then shouldn't git shows me those files were Untracked like that:

Then I created a new feature from develop branch and worked in feature/change_excel_format branch. Then git status returned: "Changes not staged for commit"

Then if I modified the file a  and type git st it will return Changes not staged for commit. So I decided to make the staged a be committed or make the not stage a to be staged, and then the previously staged file a will be discarded?

Any help would be appreciated?