drop back means move back in a position (e.g. Andrea dropped back to third place when she fell off her bike.)
drop in/ by/ over means come without appointment (e.g. I might drop in/by/over for tea sometime this week.)
drop in/ call in means visit somebody for a short time without arranging to do this. (e.g. I dropped in to see Bob on my way home)
drop off (or doze off or nod off) means fall asleep
drop out of college/ university/ a course/ a race means stop before you have completely finished a course/ race etc. (e.g. Ken went to university but dropped out after a year)
drop somebody/ something off means take somebody/ something somewhere and leave them (e.g. I have to drop my sister off at work before I come over.)