This is a Bookmark Filled 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 12,900 icons, all in the same style and quality.
It's two pages of a book almost, if you were to take a piece of paper in the very middle and make an indentation as if it's curving inwards. Make those pieces inseparable, and then open it up to make a book thing. The purpose of the icon is to place a mark on a page, hence the name bookmark. It's a play on words mostly.