In mountain biking (especially freeride) and bicycle trials a drop off refers to any manner in which you come off an object and become airborne. For example a 10 foot cliff could also be referred to as a 10 foot drop off.
Varients include sidehop drops, such as off the top of a picnic table (after riding up it of course), and wheelie drops, where one rides off (usually) a cliff on the back wheel to prevent endoing.
The main thing to remember when doing drop offs, whether 2 feet or 30 feet, is to absorb as much of the shock as possible in the knees and elbows. This prevents premature destruction of you bike. Also, it is much, much better to land on your back wheel than on your front.
Just remember, think positive thoughts and have no fear. And of course start small to build confidence and work your way up.