This is a Navigation Toolbar Left 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 13,000 icons, all in the same style and quality.
The navigation toolbar left is a rectangle shape and inside the rectangle it is divided in two sides. On the left side of the rectangle there will be other buttons or selections, which are represented by tiny circles or rectangles.