This is where straight lines near the edge of the picture appear to bow outwards, and you see this a lot with zoom lenses at their wideangle setting. It’s most noticeable if the horizon is near the top or bottom of the picture. Barrel distortion is very difficult to eradicate completely from the lens design, but it can be fixed using software, and some cameras now have distortion correction built in. It’s one of a number of common lens aberrations. Telephoto lenses often show the opposite effect, ‘pincushion distortion’.