The greater the enthalpy of formation released the greater the compound's stability
Enthalpy is the heat energy released or absorbed in the formation of the compound. This reactions can be exothermic or endothermic,
In an exothermic reaction heat is released to the environment. Think of it as a ball rolling downhill. ( or skying down a steep slope) The further the ball rolls down the harder it is to get the ball back up the hill.
So highly exothermic reactions with a large heat of formation released will be very stable
In an endothermic reaction heat is absorbed from the environment. Think of it as pushing a ball up the hill (or riding a ski lift up) This creates an unstable structure. The compound is going easily break apart releasing the energy that was absorbed.
So endothermic reactions with a large heat of formation absorbed will be very unstable.