1.
When you set off , you start a journey.
Nichols set off for his remote farmhouse in Connecticut...
I set off, full of optimism.
= set out
PHRASAL VERB : V P prep / adv , V P
2.
If something sets off something such as an alarm or a bomb, it makes it start working so that, for example, the alarm rings or the bomb explodes.
Any escape, once it’s detected, sets off the alarm...
It could take months before evidence emerges on how the bomb was made, and who set it off.
PHRASAL VERB : V P n (not pron) , V n P
3.
If something sets off an event or a series of events, it causes it to start happening.
The arrival of the charity van set off a minor riot as villagers scrambled for a share of the aid...
PHRASAL VERB : V P n (not pron)