The following ball and bowl illustrations should help in understanding parcel stability and instability. The bowl represents the 'state' of the atmosphere. The red ball represents a "parcel" of air upon which an energy is applied to the ball to initiate motion.
With the ball inside of the bowl it will return to its initial position. In the atmosphere, if a parcel returns to its initial starting elevation then the atmosphere is considered to be absolutely stable.
With the bowl turn upside down, the ball now rests on the top of the bowl. When a force is applied to the ball it begins to move on its own without any additional force applied. When this occurs in our atmosphere it is considered absolutely unstable.
On a flat surface, it a force is applies to the ball it moves. Once the force is removed the ball stops and remains in its new position. In the atmosphere, the atmosphere this considered neutral stability.
The upside down glass bowl has a slight depression wherein the ball rests. If the force is not too great the ball will return to its initial position similar to absolute stability. However if the force is strong enough the ball will move up and out of the depression and continue to move on its own. This is one of the most common states of the atmosphere called conditional instability. The atmosphere is unstable if certain conditions are met otherwise it is stable.
To measure the state of the atmosphere over our heads (temperature profile, stability, moisture, wind, etc.) we use a device called a radiosonde.