It is true that buffers prevents slight changes in pH. However, the first statement is quite misleading. So, I think the question here is how exactly a chemical buffer works. A system is a buffer if it contains a weak acid and its conjugate base. For example, acetic acid and sodium acetate is a buffer system. When an acid is added, the conjugate base reacts with it to neutralize the effect. When a base is added, the weak acid reacts with it also to neutralize.
