Phosphates come from biological waste from your fish and leftover food but they can also come from other sources like tap water. They act like a fertilizer for pest algae which can takeover a tank. Phosphates also inhibit coral growth by reducing the amount of calcium they absorb and can turn corals brown as the cells become overrun by brown algae. Hardy corals like softies can handle 0.25 ppm of phosphate, while SPS corals are sensitive to levels of 0.1ppm. To reduce phosphates you may want to reduce the bioload by feeding less or using some of the means of removing them like; more water changes, protein skimming, phosphate absorbing media, macroalage and using reverse osmosis water.
You can purchase your own phosphate level kit or get your water tested at Into The Blue!!