A rack mount is a description of a hardware device capable of being mounted in a special rack or the actual rack. Rack mounting is commonly used with large companies to hold their network servers, routers, switches, or other network devices.
The picture is an example of what rack mounts may look like in a server or network room. In this picture, equipment installation is in the beginning stages, and some racks are still empty.
See our 1U, 2U, 3U, 4U, 5U, 6U, and 7U page for information and sizes of each of the rackmount servers.