Møller-Plesset perturbation theory is a mathematical approach used in quantum chemistry to improve the accuracy of calculations for the energy levels of many-electron systems. It builds on the Hartree-Fock method, which approximates the wave function of a system, by adding corrections to account for electron correlation effects that are not fully captured in the initial approximation. The theory is based on perturbation theory, where the total Hamiltonian of a system is expressed as a sum of an unperturbed part and a small perturbation. By systematically calculating corrections to the energy and wave functions, Møller-Plesset perturbation theory provides a more precise description of molecular systems, making it a valuable tool in computational chemistry.