set off (phrasal verb)

1. to start a journey

We set off at six o’clock, and hiked through the mountains all day.

2.  cause the start of, activate

He burned the dinner in the oven, and the smoke set off the alarm (set the alarm off). 

3. enhance, show a nice contrast

The blue scarf set off her pale skin and blue eyes nicely. 

4. explode, blow up

We always set off fireworks on the Fourth of July!