xhost command in Linux

xhost 

Description
       The xhost program is used to add and delete host names or user names to
       the list allowed to make connections to the X server.  In the  case  of
       hosts,  this  provides  a rudimentary form of privacy control and secu-
       rity.  It is only sufficient for a workstation (single  user)  environ-
       ment,  although  it  does  limit  the worst abuses.  Environments which
       require more sophisticated measures  should  implement  the  user-based
       mechanism  or use the hooks in the protocol for passing other authenti-
       cation data to the server.
Learning
     Assuming we have two hosts one 'local host' and the other where connection is being established as 'remote host'  . Giving xhost + <hostname> gives permission to which host(s) should be able to establish a connection with the localhost.

Typical usage with VNC. 
     On remote host:
                                vncserver 
    Enter the password and end result is obtained as <hostname>:<display#>

  Download vncviewer clients like TightVNC, Run VNC.
connect as <hostname/ipaddress>:<display#> enter the password that was provided and the connection is established.

Now on vncserver enter xhost + or xhost + `hostname`
set display 
                  export DISPLAY=:<display#> or
                  export DISPLAY=<hostname>:<display#>

Then type
                               xclock

to validate if clock is shown and display is set right.

This is a non secure way of working but a secure connection can be established via tunneling.

Can learn about usage by using man xhost on linux/unix systems.