The ideal gas law states that the pressure of a gas and the number of moles of the gas present have a direct mathematical relationship to one another; this means that increasing the amount of gas present will increase the pressure of the gas in the container, assuming the temperature and volume are held constant.
This occurs because the gas pressure is created by the motion of the gas molecules within the container. The more frequently molecules collide with the container wall, the higher the pressure the gas exerts on the container. Therefore anything that causes more collisions within the container will increase the pressure. If you add more molecules, there are more present to collide with each other and with the container.