Sensors are great for automation, but it is also important that they do not prevent you from manually using your lights when you choose to.
Say you turn off the lights with your phone, then walk out of the room. You may not want the sensor to turn them back on before you have closed the door.

This is exactly what this delay is for. It makes the lights ignore the messages received from the sensors for a certain amount of time after they have received a manual command.

By default, this feature is enabled and set to 15 minutes