Commit
A commit is one saved version of your project, with a short note about what you changed.
In a simple example
You fix a price and save it as a commit named Update cake price. Later you can see or undo exactly that change.
Why it matters to you
Small commits with clear notes make it easy to find and undo a mistake.
Do not confuse with
A commit is saved on your computer. Pushing it sends it to the online copy.