From the original README:

Xmon interactively monitors the byte-stream connections between an X
server and a number of X clients.  Xmon recognises all requests,
events, errors and replies sent between the clients and the server
which are part of the core X protocol.  The contents of these messages
are displayed on standard output at a user settable degree of detail
from none to every bit and byte.  Xmon also allows the user to select
a number of requests or events to be monitored at a different degree
of detail.  Xmon will also block the transmission of selected requests
from the clients to the server and selected events from the server to
the clients.  Xmon also keeps statistics of the number of requests,
events, and errors received.

WWW: http://ftp.x.org/contrib/devel_tools/xmon.1.5.5.README