Stability refers to the amount and direction of the bend when you throw your disc. If you are a RHBH (right-hand, backhand), a disc that bends more to the left is overstable. One that bends to the right would be understable. Granted, this is a natural throw not one where you are trying to make it bend.
More stable = more overstable.
Less stable = more understable.
Good luck trying to remember all of this. You will be continuously tested.
As for distance, you are probably concerned with this too quickly. You want to be throwing all of your discs into a fairly tight grouping before you worry about the distance that grouping is from you.
Remember this: Control before power.