Stability defines how resistant your rocket is to fly in any direction but vertically. For HPR, it is quantified in cals, or the distance equivalent to the diameter of your airframe. 

Stability is measured from the CG, or center of gravity, of your rocket to the CP, or center of pressure. The CG is where the rocket would balance on a pole and can easily be found through testing. The CP is where on the rocket the sum of all aerodynamic forces will be acting. 

Stability is the number of cals between these two points. So, for a rocket that has a 6" airframe, a CG at 36" from the nosecone, and a CP 49" from the nosecone, it'd have a stability of 2.17 cal. In conversation, the "cal" is usually left off, and it is just said the rocket has a stability of 2.17.

The stability of a rocket should be significantly more than 1 for stable flight. Stability equal to or below will result in a rocket that tries to fly upside-down. Stability cannot be too high, either, or the rocket will try to fly horizontally.

For most NAR, TRA, and IREC launches, a stability of at least 1.5 to 2.5 is required, with faster rockets usually requiring a higher stability. The maximum stability allowed at most launches is a 5 or 6.