A server chassis is a metal structure that is used to house or physically assemble servers in various different form factors. A server chassis makes it possible to put multiple servers and other storage and peripheral equipment in a single physical body. A server chassis can also be called a server casing or server case.
Techopedia explains Server Chassis
A server chassis is primarily designed to reduce the amount of physical space consumed by a standard server. Generally, a server chassis in used in environments where multiple parallel servers are needed to work on a core business application. They typically are not designed to be connected to a display device but can be connected to a laptop/monitor for application/OS installation and maintenance.
Server chassis come in various forms, including rack and pedestal (tower). They can be classified according to their physical dimensions and are termed as being a 1U, 2U and up to 20U and more, where U represents the number of units. Typically a 1U server can house up to two servers in rack or tower form/enclosure. A server chassis can easily be upgraded/expanded to add more servers such as upgrading from 1U to 2U and above.