This is a Navigation Toolbar Bottom 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 icon is shaped like a rectangle with round edges. About a third of the way down there is a horizontal line running through it. In between the horizontal line and the base of the rectangle are two small circles followed by a long skinny rectangle.