Bell's theorem is a fundamental result in quantum mechanics that shows how certain predictions of quantum theory differ from classical physics. It demonstrates that if quantum mechanics is correct, then particles can be entangled, meaning the state of one particle can instantaneously affect the state of another, regardless of the distance between them. This challenges the classical idea of local realism, which assumes that objects have definite properties and that information cannot travel faster than light. The theorem is based on Bell's inequalities, which are mathematical inequalities that must be satisfied by any local hidden variable theory. Experiments testing these inequalities have consistently supported the predictions of quantum mechanics, suggesting that the universe behaves in ways that defy classical intuition. This has profound implications for our understanding of reality and the nature of information.