From The Apple Wiki

Carousel is essentially watchOS's graphical user interface (much the same as GNOME is to Linux, Explorer is to Windows, and Finder is to macOS) and manages graphical services such as icons and multitasking. When it is launched, it registers PurpleSystemEventPort, essentially making it the main application. Perhaps you already know about SpringBoard on iOS, Carousel is the equivalent to it for watchOS devices.

Primary Functions

  • Carousel acts as stacking window manager, rendering child applications
  • Carousel activates, deactivates and manages memory of the UIKit applications it launches