Sensors are great for automation, but it is also important that they let you use your lights manually 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 the manual override 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 (from the app, an integration, a remote control, a schedule...).
After the delay has lapsed, the automation behavior will resume.
 
By default, this feature is enabled and the delay is set to 15 minutes.

Note: if you set this delay to 0s, the devices will still apply a slight delay of up to 5s, just to confirm that your manual command has been received.