Version control means saving named snapshots of your project as it grows, so its whole history is kept. With Git, the everyday loop is three commands:

git add .                       # stage the changes you want to save
git commit -m "Add grading function"   # save a snapshot with a message
git status                      # see what's changed since the last save

Each commit is a labelled point you can come back to. Write short, clear messages that say what changed — "Fix average calculation" is far more useful than "stuff". Months later, that history is how you (and others) understand how the project got here.