It is a reduced icon of a three dimensional cube that is tilted on it's side. The cube has a diagonal line going from the top middle corner to the bottom almost left corner.
This Do Not Tilt 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 6,000 icons, all in the same style and quality.