File Explorer is the default file management application which comes bundled with every modern version of Windows. It allows the user to perform all basic file operations like copy, move, delete, rename and so on. During the development of Windows 10, Microsoft was updating folders icons, control panel icons and system app icons several times. Here is how the File Explorer icon was changed during Windows 10 development.
The first build with the new Explorer icon was Windows 10 build 9841:
The application got a dark yellow icon:
In Windows 10 build 10158, Microsoft mixed the updated icon from build 10130 with the "old" icon from build 9926, so the resulting icon had the shape of the icon from build 9926, however, it had the colors and size from build 10130's Explorer icon:
The same icon is used in Insider Preview build 14352.
While the icon was good for blending in with Universal apps in the Start menu, most users seem to have sent negative feedback about this new icon. So, in Windows 10 build 14352, the previous colorful icon has made its return: