To stand by means to wait and be ready for something to happen or for instructions to arrive. When a pilot tells passengers to stand by for takeoff, everyone should stay in their seats, buckled up, prepared for the plane to start moving. When a teacher tells the class to stand by for directions about a fire drill, students should be quiet and alert, ready to listen and act.

The phrase also means to support someone or remain loyal to them, especially during difficult times. A true friend stands by you when you're struggling with a hard problem or facing an unfair accusation. When a coach stands by a player who made a costly mistake, the coach shows faith that the player will learn and improve rather than giving up on them.

In a third sense, stand by means to do nothing while something happens, especially when you could help. If you stand by while a classmate gets teased and you don't speak up or get help, you're choosing not to act. People sometimes warn against standing by when you see something wrong.