Что означает переменная среды $ DISPLAY

От man X:

From the user's perspective, every X server has a display name of the form:

  hostname:displaynumber.screennumber

This information is used by the application to determine how it should connect to the server and which screen it should use by default (on displays
with multiple monitors):

hostname
   The  hostname specifies the name of the machine to which the display is physically connected.  If the hostname is not given, the most effi‐
   cient way of communicating to a server on the same machine will be used.

displaynumber
   The phrase "display" is usually used to refer to a collection of monitors that share a  common  set  of  input  devices  (keyboard,  mouse,
   tablet,  etc.).  Most workstations tend to only have one display.  Larger, multi-user systems, however, frequently have several displays so
   that more than one person can be doing graphics work at once.  To avoid confusion, each display on a machine is assigned a  display  number
   (beginning at 0) when the X server for that display is started.  The display number must always be given in a display name.

...

ENVIRONMENT
 DISPLAY
   This is the only mandatory environment variable. It must point to an X server. See section "Display Names" above.