The checkpoint is the so-called objective of an Escort or later half of an Assault/Escort map. It serve as an important point on the route of the payload. The final destination can be considered as a special checkpoint. There are 2 checkpoints on the Escort map and one checkpoint on the Assault/Escort map.
Anytime the payload reaches a checkpoint, the attacking team's respawn room will move to another location and there will be extra time added to their clock, in order to help them be able to push the payload to the final destination. The defending team's respawn room will sometimes also move when the payload reaches the checkpoint.
Usually the checkpoint is placed in front of a door or a gateway. Approaching that door or gateway will make the payload stop for a few seconds. Usually the reason is waiting for the door to open; and as soon as the payload passes, the door will close immediately. If the attacking team is not near the payload for 10 seconds, the payload will reverse until it touches the closed door in front of the checkpoint again.