A buffer is a term in chemistry for a solution which is composed such that it can resist pH changes. So you can add a lot of acid and the pH will hardly change (because it binds the H+ ions) and you can also add alkali and the pH will hardly change because it can also bind a lot of OH-. Generally a buffer is formed of a weak acid and one of its salts e.g. ethanoic acid and sodium ethanoate. The word comes from the idea of a 'buffer' zone which is where you leave some extra space around something so that it can safely move slightly out of position but not do any damage. There are also physiological buffers - as in your example, you can find them in blood. http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html - explains chemical buffers well