The ramp test is a method for determining the optimum focal distance between your focusing lens and the material you're cutting. The laser beam looks like an hour glass after it hits the lens in the head of the laser. You want to find the center of that hour glass. That is the focal point that you want to place either at the surface of your material or at a specific depth into the material.
To find it, you can place a block of wood that has been cut at an angle (the ramp) on the bed then engrave a straight line up the ramp. The point where the burn size is the smallest is the focal point for your laser. When engraving you will want the surface of the material to be at that height and when cutting you will usually want the center of the material to be at that height, although there are some arguments for putting it at the surface for cutting too.
If you have a powered Z axis and Light Burn there is another option. Light Burn will control the z axis and move the laser head within a range to help you find the focus. You can see more information here.