An equilibrium is essentially a balance and in terms of a buffer solution, it is a mixture of a weak acid and its conjugate base or vica versa. Its property is that the ph of this solution changes little when small amounts of strong acid or base are added to it and because it changes very little when you add small amounts of these chemicals to the solution, it allows for the ph to remain fairly constant. This in turn is important for chemical reactions to be able to proceed in living things, within a narrow optimal range. Once the ph changes too drastically, enzymes may stop working effectively or even become denatured. When this happens, enzymes lose the shape of their active sites and can no longer effectively catalzye reactions with their substrates. There is an equilibrium between the acid in the solution and its conjugate base and this is achieved due to Le Chatelier's principle. In the body, buffer solutions like the blood help to keep the correct ph for enzymes to work.