Pressure tells you how concentrated a force is. It depends on two things: the size of the force and the area it acts over. For the same area, a bigger force gives a bigger pressure. For the same force, a smaller area gives a bigger pressure, because the force is concentrated on a small patch. Spreading the same force over a large area gives a small pressure.
Sharp objects work because their area is tiny. The tip of a drawing pin has a very small area, so a gentle push from your thumb creates a very high pressure that drives the pin into a board. A knife is kept sharp for the same reason: the thinner the edge, the smaller the area, and the greater the pressure for the same force.
Wide objects work the other way round. Skis spread a person's weight over a large area, so the pressure on the snow is small and the skier does not sink. A tractor has wide tyres, which reduces the pressure on soft ground. If the area is doubled while the force stays the same, the pressure becomes half as big.
