Stdby
The Standby only works for the same day departure. You either can arrive earlier than your schedule flight or later, whichever is your case, when you arrive at the Airport, tell the agent that you would like to standby on the next departing flight. They will process your reservation and you have to wait like 15 minutes before the flight leaves for confirmation, but that happens in the gate.
Note: the stdby rules vary from airline to airline. And they have the right to deny your boarding or charge you extra. This doesn't happen a lot but it may be possible. If the flight is full, u are out of luck.