A hollow grind is concave, ground against the curve of a wheel so the blade dishes inward between the spine and the edge. It leaves very little steel immediately behind the apex, which makes it exceptionally keen and easy to touch up.
Because the sides curve inward, the steel just behind the edge is thinner than any other grind of the same stock thickness. Less material to push aside means the blade slips into material with very little effort. It is why straight razors are hollow ground and why most factory hunting knives use it.
Sharpening is easy for the same reason. There is less metal to remove to reach a fresh apex, so a hollow-ground knife comes back quickly on a stone.
Thin is fragile. A hollow grind has less mass behind the edge, so it is the wrong pick for chopping or batoning where lateral loads and impact matter.
It also binds in deep cuts. Once the blade sinks past the hollow into the thicker spine area, the concave section no longer helps and the cut gets sticky. That is a real annoyance in thick material like a heavy roast or a green log. A flat grind does not have that transition.