This parameter defines the incremental height of the macro move. In case of a line, the height value helps to build a ramp. In case of arc move, the height value helps to build a spiral.
In this example an orthogonal line is used. As you can see the tool side tilt angle is 0°. So actually the line should be on the same level like the cut slice. But with the height of 20 mm the start point of the macro is 20 mm above the actual start point.
Note that you can combine an arc sweep of 720 degrees, or even more, with a height value, to create helical ramping that can plunge smoothly into rough material.