Superposition Principle: When a number of influences are acting on a system, the total influence on that system is merely the sum of the individual influences.
According to quantum theory the resultant quantum mechanical wave function due to two or more individual wave functions is the sum of the individual wavefunctions.

Schrödinger (1935) described a thought experiment encapsulating the uneasy coexistence of the atomic and the everyday worlds. A cat is placed in a sealed box. In the box is a device that contains a small amount of a radioactive element. There is a 50 percent chance that one of the unstable atoms will decay within an hour. If an atom decays, the device breaks open a poison-containing flask, killing the cat. If there is no decay, the cat lives.

Applying the rules of quantum mechanics to everyday objects produces perplexing implications:
We cannot know if an atom of the substance has decayed and if the cat is dead or alive.
It is only when we break open the box and learn the condition of the cat that the superposition is lost, and the cat becomes one or the other (dead or alive). This situation is sometimes called
Quantum Indeterminacy--the observation or measurement itself affects an outcome, so that it can never be known what the outcome would have been if it were not observed.

Is the cat alive or dead?

According to quantum mechanics, the cat is 50% dead and 50% alive!