Plato Gravity Scale measures the amount of dissolved solids in a liquid. In the context of beer Degrees Plato (°P) measures the (primarily) dissolved sugar in the wort/beer. Measurements are done at 20°C using either a calibrated refractometer or hydrometer.
You can convert °P to Specific Gravity by applying this formula:
specificGravity = degreesPlato / (258.6-((degreesPlato / 258.2)*227.1))+1