It's what puts graphics on the screen. If you are not interested in hacking or developing it, there's not much else to know except X11 is deprecated and Wayland is not finished, leading to this suboptimal situation.
Yep and Xwayland is an X server that runs under Wayland and provides compatibility for native X11 applications, which don't natively run under Wayland.
Yep and Xwayland is an X server that runs under Wayland and provides compatibility for native X11 applications, which don't natively run under Wayland.