To cause something to start or happen. The fireworks display was set off by a remote control.
To begin a journey or trip. We set off early in the morning to avoid traffic.
To make something explode. The demolition team set off the charges to bring down the old building.
To cause someone to start doing something, especially laughing or talking. His joke set off a wave of laughter in the room.
To highlight or emphasize something by contrast. The bright red scarf set off her dark hair beautifully.