The bailer consists of a plug located at the bottom of the deck that connects to a flap on the underside of the hull where the water in the deck drains from.
Pushing a plug in and out opens and closes the draining flap. Leaving the flap in the open position creates resistance through the water, slowing the boat down, so when racing try to minimize the time that the draining flap is open.
Be very careful not to damage the draining flap when pulling the boat up onto the shore, or when pulling the boat onto / off the trailer / dolly.
If you have damaged your bailer, make sure that you check out this page.