You're right - it's a pretty basic question - but much about top bar beekeeping IS basic... that's one of the cool advantages! That, and the fact that the bees get to make their own wax - which is the MOST basic natural process, and which they need to be in charge of to support their health and the natural systems at work in the hive.
So... all that being said - here's your answer:
A top bar hive is a beehive that uses "bars" (as opposed to frames) that rest across the "top" of the hive cavity. Because the bars all touch, they make a solid surface, and while there is most likely an exterior roof as well, the bars make a sort of interior roof of their own.