This is a Enter 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.
An enter icon is a rectangle shape and between one of the sides of the rectangle there is an arrow. The arrow is going through or entering into one of the sides of the rectangle, which shows something is entering.