To write your phone number down and
A.) leave it in something inconspicuous so as to let someone whom you find attractive find it and call you
or (less commonly)
B.) literally drop it somewhere allowing anyone who finds it to contact you.
Person A: Last night I wrote my number down on a scrap of paper, folded it into a 5 dollar bill and gave it to the waitress that i wanted to hit on.
Person B: You dropped a number?
Person A: Why not, she was a hot red-head?
Person A: I like dropping my number because it's easier to meet new people who you only talked to in passing.