It's a logo for the Apple iOS operating system. The middle of the logo are the letters "i" "O" and "S" where the "i" is lowercase and the "O" and "S" are both uppercase. The letters are placed inside of a square with rounded edges.
This is a iOS Logo 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 13,300 icons, all in the same style and quality.