What's the meaning of "fall down"?

Fall down

It means drop to the ground, usually by accident.

For example:
“My friend slipped on a banana peel and fell down. I thought that only happened in cartoons!”

✨P.S.
Why fell and not fall in this example?
It's simple, fell is the past of fall.

🧠 Remember:
Whenever you're with trouble with the verb in the past or something like that, you can google it.
In this case you can search: "what is the past of fall?" and google will show you. 😉