To make a sign to show something, often using your hands or lights.
Signalling is a way to communicate without talking. It can be very important, like when driving a car.
Examples:
She signalled for help by waving her arms.
The lighthouse signalled to the ships in the night.
The coach signalled for the players to come off the field.
The cat signalled to me that it wanted to play by bringing me a toy.
The superhero signalled to the people by shining a light in the sky.
The traffic light signalled for the cars to stop.