Pauli's Exclusion Principle

For those who have heard of this but are not sure what it is, then it simply states that no two electrons can occupy the same quantum state at the same time. In other words, the quantum numbers of two fermions cannot be exactly the same time, these are the numbers used to describe the fermion.

This is seen as a fundamental law of quantum mechanics and was required to explain some of the data that puzzled those who were studying all things quantum and trying to explain the results of early experiments and the data recorded.