The Buffer is part of your camera’s memory that allows for it to process images while you continue to shoot. For many people, the amount of images that a camera can store in its buffer is not a concern. For sports photographers and other people who shoot at a high frame rate, a small buffer can cause the camera to pause while it processes the many images which were just shot.
The number of images the buffer can hold depends on if you are shooting JPG or RAW. Check your manual for details.