When an expression is in quotes, it usually indicates that it's a rather special or new compound noun, such that looking the two words up individually in the dictionary wouldn't give you a clue as to the meaning.
Web sites have "servers" which handle the incoming and outgoing traffic. As the traffic increases, more servers need to be added to handle it. (They don't make single units big enough to do the job.)
Some sites don't own their own servers, but buy the service from a farm such as is described in your sentence. - A.