What is top-tube length on a bmx?

Answer 1

It's a size measurement, taken from the center of the seat tube(the tube that the seat post goes into), along the top tube, and to the center of the head tube(the tube that the fork goes through at the front of the bike).

Answer 2

Same as for bikes in general. The top tube is the uppermost part of the frame that goes from the handlebar to where the seatpost is inserted.