A 19-inch rack is a standardized system for mounting various electronic
modules in a "stack", or rack. The rack consists of two long, thin metal
plates standing vertically in the same plane with a space of 18 inches
between them. They have holes drilled through them at regular intervals,
so that the centrelines of the holes in the two strips are 19 inches apart.
Think of a 2 inch wide door frame around an 18 inch wide door. In this case
the two plates are positioned similarly to the vertical posts of the door frame.
A second set of similar plates stands behind the first, 20 or 26 inches away.
The resulting rack looks like the four vertical sides of a large box, enclosing
a rectangle 18 by 20 inches the extra inch across the front is to the
mounting holes.

Equipment designed to be placed in a rack is typically described as
rack-mount, a rack mounted system or a rack mount chassis. The vertical
height of 19-inch rack equipment is usually a multiple of a standard unit of
height called the 'U', meaning Unit, whose dimensions were originally
developed for a horizontal rack mounting system known as Eurocard.
One U is 1.75 inches (44.45 mm). 1U, 2U, and 3U are common sizes.

In the original Eurocard system the cards were mounted vertically, standing
on end in a 18-inch long slot. This meant the weight of the equipment rested
on the bottom of the horizontal rack. With a vertical rack the weight is borne
entirely by the front and rear plates, which presents a problem when trying
to remove a chassis because all four corners have to be removed at the
same time. The common solution is to use a set of sliders mounted between
the front and rear racks, into which the chassis is slid towards the rear.
These systems typically include holes for screwing the front plate into the
rack, although this is somewhat vestigial.

This standard rack arrangement is widely used throughout the
telecommunication, computing, audio and video industries, as well as others.

BambooWeb Dictionary