Barrel distortion is a lens fault or aberration that causes straight, parallel lines in an image to bow outward, and is seen when shooting with wide-angle lenses. The wider the lens, the greater the distortion. The appearance is similar to the effect you’d see if an image was wrapped around a barrel. It can be corrected using post-capture software.
In Av mode, you select the aperture while the camera automatically selects shutter speed for a good standard exposure