In this video, Fermilab's Dr. Don Lincoln describes the principle of supersymmetry in an easy-to-understand way. A theory is supersymmetric if it treats forces and matter on an equal footing. While supersymmetry is an unproven idea, it is popular with particle physics researchers as a possible next step in particle physics.
Fermilab's Dr. Don Lincoln explains some of the reasons that physicists are so interested in supersymmetry. Supersymmetry can explain the low mass of the Higgs boson, provide a source of dark matter, and make it more likely that the known subatomic forces are really different facets of a single, common, force.