The longer version
A hold takes a slot out of the schedule for a short window, usually 5 to 15 minutes, while the player enters card details. Without a hold, two players can both reach checkout for the same court and one of them pays for nothing.
Holds matter most at peak times, when the Tuesday 19:00 court has several people looking at it in the same minute. They also matter for open play and classes, where the last spot is the one everyone wants.
The mistake is a hold that is too long. A 30-minute hold on a popular slot means an abandoned checkout blocks a real booking. Staff should also be able to see holds from the desk and clear one that is stuck.