This is a Restore Window 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 13,000 icons, all in the same style and quality.
There is a square with two other squares overlapping each other in the center of it. The two overlapping squares also have a line towards the top of them. The square to the right is overlapping and higher than the one on the bottom and left.