It simply means that if you, through no fault of your own (they usually require you to do some debugging, i.e. switching rj45, bypassing your router, and so forth), cannot reach the guaranteed speed when measured to a reference server, they'll fix it. (Oh, and it probably has to happen with some regularity - I don't think they'll send a technician if you got 47 Mbps just the once.) I actually don't know what happens if they can't, I've never seen that happen. You'd probably be able get a refund, at a minimum. Now, while they do oversell bandwidth, it's my understanding that this mainly applies access outside each providers' own net. I.e. you should be able to max out your bandwidth to the reference servers (commonly the ones reached through bredbandskollen.se). Don't quote me on this though, I'm sure there are other slashdotters with better knowledge of this.
If all their customers was always maximizing their bandwidth, my guess is that the policy would change, or rates would spike. There are no caps in place on regular broadband right now, that I know of. Though, if memory serves, the mobile broadband providers have caps.