Kohn's Theorem is a principle in quantum mechanics that relates to the behavior of electrons in a system. It states that the ground state energy of a many-electron system can be determined solely from the electron density, rather than the many-body wave function. This means that knowing how densely the electrons are distributed in space allows us to calculate the lowest energy state of the system. The theorem is significant because it simplifies the study of complex quantum systems. By focusing on electron density, researchers can use density functional theory (DFT) to make calculations more manageable, leading to advancements in fields like materials science and chemistry.