Fusion is the opposite of nuclear fission which is the reaction powering traditional nuclear plants.
Fission: What traditional nuclear power plants do. It involves splitting a heavy atom (like Uranium) into smaller pieces. This process creates a chain reaction that must be carefully controlled and produces radioactive waste that stays dangerous for thousands of years.
Fusion: What OpenStar is building and what powers the Sun. It involves fusing two light atoms (like Hydrogen) together to make a new one (like Helium). It cannot melt down, and produces no long-lived high-level radioactive waste.