This is a Bookmark 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 12,600 icons, all in the same style and quality.
The image appears to be a flag or banner. The basic shape is a rectangle with the long sides standing vertically. At the bottom instead of a straight line to close the rectangle the center point of that line dips up like a triangle is missing from the bottom of the rectangle.