Basically it means what it says...

You should not try to use -sv -cl on your hypothetical server for a MMOG. The addition of the -cl will cause it to run quite slowly when it starts having to handle data from all the clients. What they are suggesting you do is use a dedicated computer to run the server (-sv only) and then conect to it with the clients (-cl). In doing it this way, you can cut down on the stuff that the server does not need to see (an fx that are just for show for example).

In theory you could have an entirely seperate set of models and textures for you server to give it even less work so it can focus on handling only what it needs to (being a good little traffic cop).

Just a heads up though, MMOGs are probably one of the hardest things to get up and running. You might want to put that on hold until you are more familiar with the game engine itself.