Running a web farm requires you to provision and manage multiple servers.  Running a web farm involves (among other things):

Installing IIS, ASP.NET and all of the core platform components on the servers

Installing and configuring custom IIS modules (UrlRewite, Media Services, etc)

Configuring IIS Application Pools and Sites

Setting up SSL certificates for things like HTTPs endpoints

Copying and synchronizing the appropriate sites/applications/content across the various boxes

Coordinating the various web servers with a HTTP load balancer to distribute load

Administrators and developers managing a web farm today often perform a lot of manual steps to do the above (which is error prone and dangerous), or write a lot of custom scripts to automate it (which is time consuming and hard).  Adding new servers or making configuring or application changes can often be painful and time-consuming.

The Microsoft Web Farm Framework makes this process much easier, and enables you to manage web farms in a completely automated way. Best of all, it is really easy to setup and use.