It simply means a computer cluster- it is the user of huge number of powerful servers that has being pre-installed with a computer software which includes the likes of Cinema 4D, 3Ds Max, After Effects etc. your scene numbers will be broken down into different available servers on other render farm, each render farm has been pre-configured to render its own portion in line with other servers.
This means, you can a project that is very large to be render by over 20, 40 or 60 servers in the space of 1 or 2 hours instead of your personal computer. The rendering of any project always takes only a part of the render farm servers: from 1 to 100, because it is inefficient to run the render on 800 or 1000 servers.