The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction. If the conditions are changed, the amounts at equilibrium can change too.
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. The system does not get pushed further and further by the change. It reacts in the way that opposes it.
For example, if some product is removed from a system at equilibrium, the system is no longer at equilibrium. To counteract the change, the system makes more product. If more reactant is added, the system counteracts the change by making more products, which uses up some of the added reactant.
At GCSE you use Le Chatelier's Principle to make qualitative predictions. That means saying which way the equilibrium position shifts, and whether more products or more reactants will be present, when you are given enough information about the reaction.