Take the reversible reaction A + B ⇌ C + D at equilibrium. 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. Add extra A, and the forward reaction speeds up. More A reacts with B, so the concentration of B falls and the concentrations of C and D increase. The position of equilibrium has shifted towards the products. This follows Le Chatelier's Principle, because the system acts to use up the extra A.
If the concentration of a product is decreased, more reactants will react until equilibrium is reached again. Remove some D and the reverse reaction becomes slower than the forward reaction, so A and B react to make more C and D. Taking a product away as it forms keeps the reaction going and increases the amount of products made.
The opposite changes work in the same way. Adding more product shifts the position towards the reactants, and removing a reactant does the same. To predict the effect from given data, find which substance has changed, then decide which reaction must speed up to oppose that change.