Think negative feedback of biology. A system at equilibrium will PARTIALLY OPPOSE any changes to this equilibrium.
I say partially oppose for a reason. Let's say you initially had 10 moles of nitrogen gas in equilibrium with 5 moles of hydrogen gas and 10 moles of ammonia. If you then dump another 5 moles of ammonia gas into this system, the nitrogen concentration will increase, prompting the system to oppose that change by reducing the nitrogen concentration with a reaction of ammonia to reform nitrogen. However, no matter how the system tries to reduce the nitrogen concentration, the nitrogen concentration at the end will always be greater than the initial concentration of 10 moles/whatever volume. See why it's partially opposing?