This icon is made of 3 rectangles stacked on top of each other like you might stack books on a shelf. Each rectangle has a small black dot in the right corner. The icon is meant to represent things stacked on top of each other.
This Stack 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 6,100 icons, all in the same style and quality.