A buffer solution is an aqueous mixture of an acid and its conjugate base which resists changes in pH due to the addition of a strong acid/base.
Yeah like jmm said, its a solution which resists changes in pH. If you add acid to a normal solution, its pH will go down significantly. If you add acid to a buffer solution, its pH won't change. So if you're performing a reaction but you need the pH to stay the same, you could buffer the solution. For example you could add acetic acid and sodium acetate to the solution which when combined at a particular ratio, will act as a buffer.