Can someone explain to me how Super Smash Bros. is played?

My son has had several incantations of this game and I’ve seen it played by others but I still can’t tell what the objective of the game is. (yeah, I know it’s to beat up your opponent).
What does the “%” number represent for each player? I was told it’s not a life meter but rather vulnerability number and the higher it is the more vulnerable you are?
Are the matches timed or do you play until one player is left standing?
Is the objective to inflict damage on your opponents or just knock them off a platform?
Is falling off a platform the same as someone knocking you off?
How does somebody “win” or “lose”?

He’s of course getting the newest one for Xmas and I want to play also but games nowadays come with no instructions and having him try to explain it to me is useless.

The main goal is to knock the other player off the screen (either side, top, or bottom). The percentage is how much damage you’ve taken. The more damage you’ve taken, the farther you get knocked with equivalent hits.

There’s several game modes, so you can have a timed battle with infinite lives or an untimed battle with a set number of lives.

Falling off the screen on your own makes you lose a life, but in general the other player won’t get credit for it. In timed modes you usually get a point for every time you knock a player off the screen, and lose a point for every time you get knocked off the screen.

You might watch some Youtube videos: