Take a reversible reaction at equilibrium: A + B ⇌ C + D. If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium. The concentrations of all the substances then change until equilibrium is reached again.
If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again. So if the concentration of A is increased, more C and D are made, and some of the extra A and B is used up.
If the concentration of a product is decreased, more reactants will react until equilibrium is reached again. So if some of C is removed, more A and B react to make more C and D. The system is working to counteract the change.
The same logic works the other way. If the concentration of a product is increased, the reverse reaction is favoured and more reactants form. You may be given data about a reaction and asked to predict what happens when one concentration changes.