After you have added files to your repository and/or changed old files, you get to a point where you want to make sure the actual state is saved, so that you can go back to that state in the future if you need - that is called commit. That's one of the reasons for using Git.
In order to do that, you run: git status
Which shows the modified files and/or new ones.
You choose which files you want to commit by running: git add filename.ext
You might as well add whole directories: git add directory/
If you run 'git status', it will show you which files were already added to the commit queue.
When you are finished with choosing files to commit, run: git commit
In the first line type a brief description (think of this as the subject of an e-mail), add a blank link, then type the full description of the commit.
Once you save and close the editor, the commit will be applied. You can get the list of commits by running: git log