Equipment racks are most commonly 19-inches wide. This is a legacy from the telephony industry in the USA from the 1920's.
The equipment installed in racks commonly has heights measured in Rack Units; abbreviated as U. 1U = 1.75 inches. For example, a given equipment module may be said to be "1U" high.
These are part of a good structured cabling plan. They are a set of RJ45 sockets, with punch-down rear connections. Building cables are terminated into these RJ45 punch-down sockets. Then short patch leads complete the connection between the patch-panel socket and the terminating equipment (e.g. a LAN switch).
Often, there is equipment that can not be directly mounted in a 19" rack, but is still required to be located with other rack-mounted equiment. Equipment shelves solve this problem. They are simple rack-mountable shelves on which smaller equipment can be installed; possibly secured with cable ties or similar.