man X
查看X的手册里面有详细说明
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 efficient 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.

screennumber
Some displays share their input devices among two or more moni‐
tors. These may be configured as a single logical screen,
which allows windows to move across screens, or as individual
screens, each with their own set of windows. If configured
such that each monitor has its own set of windows, each screen
is assigned a screen number (beginning at 0) when the X server
for that display is started. If the screen number is not
given, screen 0 will be used.