A buffer is something that prevents something else from being harmed or that prevents two things from harming each other.

The buffers on a train or at the end of a railway line are two metal discs on springs that reduce the shock when a train hits them.

A buffer is an area in a computer's memory where information can be stored for a short time.

If you say that someone, usually a man, is an old buffer, you mean that you think they are rather foolish.