1.
If something falls off, it separates from the thing to which it was attached and moves towards the ground.
When your exhaust falls off, you have to replace it.
= drop off
PHRASAL VERB: V P
2.
If the degree, amount, or size of something falls off, it decreases.
Unemployment is rising again and retail buying has fallen off.
PHRASAL VERB: V P
see also falling-off