The crampon attaches to the mountain boot. It is a many spiked device. These spikes find purchase in the ice, just by being so sharp. Often modern crampons will have large spikes protruding forwards. When climbing a vertical face of ice one kicks ones foot into the ice. These front points grip the ice and one stands up on them. This is referred to as front-pointing. Before front-pointing was developed people had to manually cut steps into the ice.

I heard of some guy climbing in the Alps. He mantleshelfed onto a ledge, but stood on his hands with his crampons. I think he promptly fell off and had to be air-lifted out of the mountain