Idea of Bookmark Icon
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.
This page contains the Bookmark vector icon, as well as variations of this icon in different visual styles, and related icons. All icons are in the flat vector style, however, differ by the line thickness, fill, and corner radius.