Logo iOS and other 74,400 icons from Icons8 icon pack follow the visual guidelines of the operating systems: Windows, iOS, Android KitKat, and Material. PNG format is free up to 100x100 px. Icons are availble for download in PNG and vector (SVG, PDF, EPS, and more).
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.