Package xorg-xnest: Information

Xnest is an X Window System server which runs in an X window.
Xnest is a 'nested' window server, actually a client of the
real X server, which manages windows and graphics requests
for Xnest, while Xnest manages the windows and graphics
requests for its own clients.

You will need to install Xnest if you require an X server which
will run as a client of your real X server (perhaps for
testing purposes).