This icon is used a lot to verify you are not a robot. It is a reduced rectangular box that is filled with a white background and floating letters and numbers inside it.
This CAPTCHA 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 6,100 icons, all in the same style and quality.