Good history reads like a changelog: each commit does one thing and says so. That makes it easy to review, easy to revert, and easy to understand a year later.
Here you've fixed a bug in app.js, tweaked some styling in styles.css that isn't finished, and scribbled notes.txt that should never be committed at all.
git status sorts them for you. Modified files are ones Git already tracks. Untracked files are ones Git has never seen — it won't include them in anything until you git add them.
Try it
Commit the bug fix on its own. The styling stays in your working directory for later; the notes stay untracked.
git add . stages everything in the folder. It's convenient — and it's exactly how notes and secrets end up in history.