I will illustrate my answer with two pictures:
Healthy goldfish, erect fins
Sick goldfish, clamped fins.
An average healthy goldfish should be swimming around in a balanced way with all fins erect and no difficulties in moving or maintaining buoyancy. Now, if a fish is sick, you'll see that it's putting the fins closer to it's body and keeps them that way, usually combined with an unwillingness to swim.
Healthy fish can sometimes put their dorsal fin down closer to the back while they're speeding forward, but you'll notice that they soon put them back up again like sails!
Clamped fins show that a fish is uncomfortable either because of the water or some kind of infection, in other words, they're stressed, and a fish usually doesn't have "half-way clamped fins" but if they're uncomfortable, you'll notice them being far less energetic! You can see that the fish on the second pic looks rather lethargic, while the first one is wiggling himself to madness.
They don't clamp the fins unless there's something wrong, simply, it doesn't matter how "severe" the problem is.