ResolutionA web farm is a number of servers offering a web service acting as a single service.
If you put three web services into a single web farm, when a domain is added into Helm the site will be created on all three servers.
Most hosts will not use web farms. If you are unsure, do not use this feature.
In order to have web sites distributed evenly across servers, you will only need to add the services into the same resource and use this resource for the relevant plans and packages.