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 is a Add Folder icon. We've made it in iOS style
, first introduced in iOS version 7 and supported in all later releases up until now (at least iOS 11). This style is based on thin two-pixel lines and is optimized for 50x50 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 13,300 icons, all in the same style and quality.