The Pauli Exclusion Principle states that, in an atom or molecule, no two electrons can have the same four electronic quantum numbers.
What this means in simple terms is that electrons “share” an atomic orbital by having opposite “spins”. You might think of it like bunk-swapping on ships to minimize idle crew space.
The “spin” analogy here being that TWO seaman “share” the bunk and work cycles on “opposite” sides of the shift. They cannot BOTH be in the bunk (or on duty) at the SAME time.
This theory is upheld by observations of atomic orbital interactions and atomic structure. But as with all theories, remember that it is just the best workable explanation that we can come up with to explain our observations. So far, it works quite well.