1.
If someone in authority lets you off a task or duty, they give you permission not to do it. ( mainly BRIT )
In those days they didn’t let you off work to go home very often.
PHRASAL VERB : V n P n / -ing
2.
If you let someone off , you give them a lighter punishment than they expect or no punishment at all.
Because he was a Christian, the judge let him off...
When police realised who he was, they asked for an autograph and let him off with a warning.
PHRASAL VERB : V n P , V n P prep / adv
3.
If you let off an explosive or a gun, you explode or fire it.
A resident of his neighbourhood had let off fireworks to celebrate the Revolution.
PHRASAL VERB : V P n (not pron) , also V n P