Pressure only affects a reversible reaction at equilibrium when gases are involved. In a gas, pressure comes from the molecules hitting the walls of the container. The more molecules there are in a given volume, the higher the pressure.
To predict what happens, count the molecules of gas on each side of the balanced symbol equation. For the reaction N2(g) + 3H2(g) ⇌ 2NH3(g), there are four molecules on the left and two molecules on the right. The left is the side with the larger number of molecules and the right is the side with the smaller number of molecules.
This follows Le Chatelier's Principle: if a change is made to a system at equilibrium, the system responds to counteract the change. An increase in pressure shifts the equilibrium position towards the side with the smaller number of molecules, because that lowers the pressure. A decrease in pressure shifts the equilibrium position towards the side with the larger number of molecules, because that raises the pressure.
So in the ammonia reaction, increasing the pressure gives a higher proportion of ammonia in the equilibrium mixture, and if the pressure is decreased the proportion is lower. If both sides have the same number of gas molecules, as in H2(g) + I2(g) ⇌ 2HI(g), a change in pressure has no effect on the equilibrium position.