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 flat color style
. This format was developed by Icons8 designer team and became immensely popular for use in web and mobile applications. Feel free to download this icon in PNG format for free, or unlock other formats for $5. Unlike other icon packs that have merely hundreds of icons, this monster icon pack contains 5,900 icons, all in the same style and quality.