Hartree-Fock Theory is a method used in quantum chemistry to approximate the behavior of electrons in atoms and molecules. It simplifies the complex interactions between electrons by assuming that each electron moves independently in an average field created by all other electrons. This approach helps in calculating the energy levels and wave functions of multi-electron systems. The theory is based on the Pauli Exclusion Principle, which states that no two electrons can occupy the same quantum state. By using this principle, Hartree-Fock Theory provides a way to account for electron correlation, although it may not capture all interactions accurately, leading to further refinements in computational methods.