It's a relief grind, making sure you sharpen at the same angle every time.
When a knife is made, the blade is shaped with two bevels. The primary bevel gives shape to the whole width of the blade. Most common are hollow grinds and flat grinds, occasionally you see a convex grind.
The secondary bevel is what shapes the edge, determines it's angle and thickness.
If you always match the secondary bevel (the original edge angle) when sharpening, you won't have any problems. However, it's much easier to sharpen at a slightly steeper angle, as you'll only be removing metal from the the actual edge, and not the entire secondary bevel. If you sharpen at that slightly steeper angle long enough, you will eventually grind off enough metal to have re-set the entire secondary bevel to that angle, and have to sharpen at a angle slightly steeper than the last.
Wash, rinse, repeat.
This story ends with an incredibly obtuse edge that is impossible to sharpen and doesn't cut anything.
Until, you go and re-set the secondary bevel to it's original angle, giving you a nice thin edge that's easy to sharpen again.
You can go through that cycle continually, or make sure to grind back the secondary bevel every time you sharpen, keeping the edge thin enough to be easy to sharpen.
In other words, grinding at an angle slightly shallower than your sharpening angel forms the back bevel, or relief grind.
