Roleplaying is just that, playing a role. It's a very broad term that means more than just sitting in an over-glorified chatroom playing pretend pals with likeminded people.
A lot of servers have "rp" slapped on them just as a way to get around the whole 'making the server fun to play' thing. A lot of rp servers are pretend-play chatrooms. Then again, there have been a fair few roleplay servers that actually have lots of stuff to do.
The only issue is that they require both effort from the creator(s) of the server to make the roleplay engaging and interesting, and some effort from the players to get into the server beyond walking around and killing everything.
Roleplaying in itself isn't something that should make up the majority of a server's content. But then again, if all you want to do is build and/or shoot people, maybe you should play on a different server.