GroundLock

Ground lock is a fancy way of saying that the texture/feature that has it enabled will be placed in world space. In other words, as you walk around, the texture will "stand still" as if it were part of the floor and the shadow is just a way of "looking into" the texture.

So you can use this to create fun scrolling effects where the scrolling only happens when you move around, indicating there's a greater world or a portal that you're walking on.