The ClrDraw command clears away anything drawn on the graph screen — that is, the result of any of the graphics commands, except for Graph (which you can only clear with ClrGraph). It also leaves alone any functions or plots (which are disabled by FnOff and PlotsOff, respectively), as well as axes, labels, a grid, etc. (which can be disabled by the setGraph() command).
Be warned that it doesn't update the screen. For example, if you run the following program:
:circ() :Prgm :Circle 0,0,5 :ClrDraw :EndPrgm
it will draw a circle and then end on the graph screen with the circle still drawn. The screen will actually update the next time you change something on the graph screen; you can also use DispG to update it (although in the program above, DispHome might be more appropriate).