The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction. Change the conditions and the balance between them can change.
If a system is at equilibrium and a change is made to any of the conditions, the system responds to counteract the change. This idea is called Le Chatelier's Principle. In effect the system tries to undo whatever has been done to it. If a change would increase the amount of a substance, the system acts to reduce it. If a change would decrease the amount of a substance, the system acts to increase it.
The change disturbs the balance of the forward and reverse reactions, so the system is no longer at equilibrium. The amounts of reactants and products change until equilibrium is reached again, at a new position of equilibrium. We say the position has shifted towards the products or towards the reactants.
You can use the principle to make qualitative predictions. That means saying which way the position of equilibrium will shift when you are given enough information about the reaction, not working out by exactly how much. The conditions that matter include concentration and temperature, which you meet in the next two lessons.