Wikipedia has a Garbage time article that defines it as follows:
Garbage time is a term used to refer to the period toward the end of a timed sports competition that has become a blowout when the outcome of the game has already been decided, and the coaches of one or both teams will decide to replace their best players with substitutes. This serves to give those substitutes, who are usually less experienced or younger players, actual playing experience, as well as to protect the best players from the possibility of injury.
Garbage time owes its name to the fact that this period in a game is frequently marked by a significant drop in the quality of game play...
Magic: the Gathering can similarly have games where the winner is effectively already decided but neither player has officially lost.
Players are often expected to concede in situations like that, and in many cases it is strategically a good option for them to do so, but the rules do not require them to do so. As long as a player is progressing through their turn and taking individual game actions in a timely manner, they are not in violation of slow play rules.
