xvfb-run is a wrapper for the command xvfb which simplifies the task
of running commands (typically an X client, or a script containing a
list of clients to be run) within a virtual X server environment.

xvfb-run allows for instance to listen to a media through firefox
in the console mode, instead of in a graphical environment.

Simple example:
xvfb-run -a firefox <some web page with an embedded song>

It can also be used from a terminal in a graphical session, but then in
the previous example firefox should not be already started.

To now more read the man page:
man xvfb-run
Or just type
xvfb-run --help

Didier Spaier, 17 December 2018