PAR (Photosynthetically Active Radiation) measures the amount of usable light reaching your corals — the honest way to judge coral lighting, since watts and brightness don't tell you what a coral actually gets.
Different corals want different PAR: soft corals and most LPS around 50–150, SPS 200–400. PAR falls off fast with depth and varies by fixture, so many reef clubs lend a PAR meter — and you always acclimate corals to more light slowly.