It's an image of a long diagonal line from the top left to the bottom right. It's cutting a signal image in half diagonally. The signal image is a small circle in the middle with two partial curved lines emanating from each side.
This Offline icon is made in Material Design style
developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 5,500 icons, all in the same style and quality.