Grand Unified Theories: three of the four fundamental forces we know of, namely electromagnetism, the weak interaction, and strong interaction, share very similar mathematical structures (local gauge theory) that lead particle physicists to believe that they have a common origin in a larger theory. Such a "unification" of the electromagnetic and weak interactions into an "electroweak" interaction is known to happen, resulting in parameters such as coupling constants and masses governing the two interactions to be inextricably connected. "Grand Unification" or "Grand Unified Theories" (GUTs) posit that there is further unification such that the strong and electroweak interactions arise from a single gauge group with one coupling constant.
Proton Decay: A generic consequence of GUTs is that they predict that the proton is an unstable particle that should eventually decay, albeit at an extermely slow rate. A few generations of experiments have searched for proton decay with successively larger detectors, culminating in Super-Kamiokande, where decays of protons in 22.5 kilotons of water have been looked for and not found. This places the lifetime of the proton at at least 1033 years.
Serach in LAr: with similar size detectors, DUNE will search for proton decays with Liquid Argon Time Projection Chambers (LArTPC), a detector technology that allows fine details of the particles to be studied. This may allow us to search for new proton decay modes with better sensitivity.