In the most simplified explanation possible, acids are substances that dissociate to release protons in solution. Bases bind to protons because they have available electron pairs.
PH is a measure of the proton concentration of a certain solution. It is the negative logarithm of the concentration so a low pH indicates a higher concentration.
A buffer system in a solution resists extreme pH change by absorbing added protons or basic ions. A weak acid or base and its salt can be used to make a buffer, and the buffer pH can be adjusted by adjusting the concentrations.
The optimum pH of human blood is around 7.4. Blood uses multiple buffer systems to maintain that pH, but the most important involves bicarbonate and carbonic acid. These two molecules respond mainly to carbon dioxide and lactic acid.
Buffers are extensively used in laboratory research to keep pH at a desirable level, especially when a byproduct of the reaction is an acid or a base.