A drop-off is basically a step or drop
down. Like a jump but with no lip or projected ramp and
on the same level as the approaching run.
The most common type of drop-off is the "log drop" one of these will be found on just about every course you'll ever do.
It consists of a log dug in across the track with the top side filled in with earth and rocks to make a step.
Some riders bunny hop them, others slow and wheelie drop, but the easiest method is to hit them with a bit of speed with your weight back.
Well it works for me. Or there's always the chicken run. (see jumps)