This Open in Popup icon is made in Material Design style developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 5,300 icons, all in the same style and quality.
This is an icon to indicate that a separate smaller window will open. There is a rectangle with rounded corners. In on of those corners there is an arrow with the head pointing to the corner directly diagonally.