Have you ever tried to plane a piece of wood?
We all know that wood has a grain that goes only one way, and when you try to plane with the grain you get nothing. The Plane runs across the wood and slices off little wood with little resistance.
However, when you Plane against the grain it suddenly becomes more difficult, and the bladed edge starts slicing and chipping pieces of wood off all over the place, creating a real mess. Push too hard and you'll cut too deep into the wood, and likely damage your Plane or leave a bad mark on the wood.