The "Choose a Slice" window shows a gray wireframe representing the domain.  In this domain are a yellow and an orange hotspot and a red line determined by these hotspots.  The orange hotspot moves the line while the yellow hotspot rotates the line.  The slice curve calculated will be calculated along this line.

The "f(x,y,z)" window shows the domain colored according to the value of the function.  This window also shows the line from the "Choose a Slice" window colored according to the value of the function.

The "Slice" window shows the value of the function on the vertical axis plotted against the length along the line on the horizontal axis.  The orange hotspot corresponds the 0 on the horizontal axis, and the yellow hotspot is in the positive direction along the horizontal axis.