This is a type of Totals market is a way of backing the total number games or sets there will be in a given match. The sportsbook will predict how long they think the match will last and you, the bettor, decides whether they think there will be more or less games or sets than the total specified.
If you think a game will go all the way, then go high with an Over bet, but if you think it will be over in the flash then go low (Unders).