A render farm is a high-performance cluster of computers that distributes the process of rendering CGI (computer-generated imagery). Modern photorealistic CGI requires complex optical simulations, which lends itself to distributed computing techniques.
The distribution of rendering work is controlled by a program called a queue manager which controls the assignment of render jobs and the management of software licenses. During the render, a computer may be instructed to render a single image, a set of images, or a small part of a larger image.
Note: Dedicated technicians who manage and maintain render farms are colloquially referred to as "render wranglers."