In model rocketry you must ensure your rocket is stable. A stable rocket will fly perfectly, while an unstable rocket will fly in an unpredictable pattern possibly ruining the rocket, damaging property or hurting someone. Stability is extremely important when building a rocket from scratch, and a good idea to know how stable your kits are also.
CP and CG
The Center of gravity (CG), also known as the center of mass, and the center of pressure (CP)