A Game Over is a common term in video games, usually signifying the player losing.
When the player loses a fight, they are treated to a defeat screen, whose appearance varies based on the game. The game will then cut to a countdown. In most cases, the announcer counts down from "9" to "0", and the player has that much time to insert more coins to continue (or in other cases, press "Start"). The countdown can be sped up by pressing buttons other than the "continue" button(s).
If the player continues, their character will be "healed", or recover, and be able to fight again; alternately, the player can choose a different character. If the player does not continue, they are treated to the Game Over screen, and can submit their initials for the hi-score board, depending on the game.