Configuration Interaction (CI) is a quantum chemistry method used to describe the electronic structure of molecules. It involves considering multiple electron configurations, or arrangements of electrons, to account for their interactions. By combining these configurations, CI provides a more accurate representation of a molecule's energy and properties compared to simpler methods. In CI, the simplest configuration is the Hartree-Fock solution, which is then expanded by including additional configurations that differ by the excitation of electrons. This approach helps capture the effects of electron correlation, leading to improved predictions of molecular behavior and reactivity in various chemical processes.