What does $DISPLAY environment variable mean

What does $DISPLAY environment variable mean if the value is greater than 0 on my local machine?

1 Answer

From 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.