Abstract: This paper introduces three famous paradoxes of quantum mechanics concerning the relationships between quantum mechanics and local reality, contextual reality, and macroscopic reality. The three paradoxes correspond to three thought experiments, from which the inequalities derived from the hidden variable theory and reality were developed. These inequalities can thus quantitatively evaluate the relationship between quantum mechanics and the three realities, making entanglement and other quantum properties become real resources that can be detected and utilized, and so widely used in quantum cryptography, quantum teleportation, and quantum computation. The emergence of quantum information theory has further driven the generation and development of quantum communication, simulation and sensing, as well as other high-tech fields, thus opening the "second quantum revolution".