This is a phrase used in baseball to refer to a re-scheduled game. It can also be used to apply to the same situation in other sports, though it is more common in baseball because of the outdoor nature of the game. There are many reasons a game may have to be re-scheduled, including: weather, natural disaster, scheduling conflict, or some other unforeseen event. When a game is initially cancelled, a new date is usually chosen as soon as possible in order to ensure the game can still occur within the time allotted for the regular season or playoffs, whichever the case may be. When a game is cancelled in the major leagues, it needs to be rescheduled to ensure every team plays the same number of games.
The phrase, "to make up," means to fix, replace, or to otherwise make whole again. When a game is canceled and then re-scheduled, the new game replaces the old game. Usually, the first game will either have never happened, or been called off so early in the game that not much of the game was played. Currently, Major League Baseball mandates that five full innings of at-bats per team must be played in order for a game to be called without the need to re-schedule. In this case, if the home team is losing, the game may be called in the top of the sixth inning.