for "practical" purposes this is a constellation diagram.
(This is not supposed to be anything but a way of describing what this is , and for the purists out there, I am talking only n "practical" terms for a newbie, so no flames please )
It represents the "quality" of the data that is received.
In this case, imagine that someone over a wall has to write a number 1-4 on an apple and throw it over the wall. Your job is to try to catch the apples and put them in one of four buckets , depending upon the number on the apple.
If all goes well all the apples will be sorted into 4 buckets in the end.
Now if the guy on the other side has bad writing or the number is unclear, or he throws six at a time & they hit the ground and become bruised and hard to read, then you have to decide which bucket it goes into, with varied degrees of confidence. and some may be wrong, some may just have to stay on the ground etc.
OK.. so the diagram above is the 4 buckets and the "x's " are apples. If the signal was 100% perfect all the x's would be in a pile in the 4 spaces, ( like Zoltan's example with 2 "buckets" and dots) . In real life with a weak noisy signal , the x's are all over the place, but the "smarts" in the outernet receiving software can make a lot of sense from even a pretty poor signal.
I hope that gives you an idea of what you are looking at there. Of course the reality of it is a lit more complicated, but I always find analogy better that maths