It's a very simple question with a very long and complicated answer. See me tomorrow at Harbin and I will explain everything to you in a detail that may have you looking like a deer caught in the headlights...but a simple answer is:

When thrown flat for a right handed person...with perfect technique....stable means it flies reasonably straight as an arrow....understable means it will fly to the right...either as a nice slow turnover...or to the extreme that it flips hard...hits the ground on its edge and becomes a roller....Overstable means that it will go left....either as slow turning hyzer...or to the extreme of a hard left meat hook.

Different plastics cause variations in how the wind moves over the disc in flight and how that flight plate reacts and moves...sometimes because one plastic is actually a different shape than another because of the cool down process at the factory....the color of the disc can also cause different stabilities because a blue coloring will cause the disc to cool at a different rate than a red coloring additive and thus slightly alter the shape and/or the exterior texture.

Weights affect stability because it is harder to throw a 174G driver as fast as a lighter driver...speed is the factor here...how fast that air is flowing over that airplane wing we call a disc...also different weight disc may have plastic thicker or thinner at different parts of the disc causing slight alterations to the gyroscopic forces....

Anyway...disc golf is a lesson is Physics....where your knowledge expands the more you play.

as i understand it, stability isn't necessarily about how hard a disc fades, although more stable discs do tend to fade more. instead, stability refers to a disc's tendency to want to get down to the ground or stay up in the air. hyzers/anhyzers can be thrown regardless of a disc's stability, however understable discs tend to hang in the air longer and get more d. if you need to throw a big sweeping hyzer, it can be done with an understable disc just as well as it can with a stable one; the understable one willl just go further.

glide, speed, turn, and fade are the four categories rated for a disc's flight. how much a disc flips or comes in at the end is turn and fade. stability is something different.

 As a beginner looking for an ideal disc, you should know that there is no such thing as "stable" as described by Discraft. For example, a Comet is a "Stable Mid-Range Driver". What they mean by, "stable" is that it flies relatively straight, (in beginner terminology) so they refer to a Wasp or Drone as overstable. If you were to ask anyone who throws a Comet, "Is the Comet a stable disc?" they would most likely tell you that it's one of the least stable mid range discs in their bag.