AV stands for aperture value, which makes the aperture the priority. This mode is basically for controlling Depth of Field. The larger the aperture, the shallower the depth of field. A shallow depth of field makes the main subjects standing out. As the aperture becomes larger, foreground and background start to blur.
Examples of AV:
My own images:
To take these images, I used an aperture of around f/2, which blurs most of the background, making the focused object the dominant part of the image.
Contact sheets: