If you tiled it in the world and rendered it, the camera rays would look something like below, but your triangle-count would be huge:
So instead, in this technique you just pick the direction that the camera is looking in (or some other arbitrary choice of direction that looks good), and render a single "tile" of grass to a texture, using an orthographic projection in that direction.
Do you have to render a new "tile" for each angle of sloped terrain, since the camera is looking at the grass at a different angle?