It's a drawing of a file folder with a plus sign enclosed in a circle covering the lower right corner. It is similar to the symbol for "add folder" in the Windows operating system.
This Add Folder 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 5,700 icons, all in the same style and quality.