It's a little difficult to describe, maybe someone will be able to describe it better but the way I see it is as if you were to take a slice out of a mountain, it'll show you that slice in black. The easiest way to start seeing it this way is to fly through valleys. It's not super useful just for flying over hilly terrain imo, but rather flying through relatively wide valleys in the dark. The black will represent a slice of the sides of the cliff, and the green in the middle will be the valley through which you can pass.
See page 112 of the manual for more info, but roughly, Log (default) is good for making out targets on the ground, in mixed terrain, and Lin is better to sharply make out terrain features. Both amplification modes can be adjusted using the MKR knob.
The manual says it gives you a close-up of an area on the radar for making out details more clearly, but exactly what area it is or how you adjust it I have never been able to determine. Perhaps it's a work-in-progress feature, or maybe someone else will chime in.