Above it the oil is undersaturated: one liquid phase, and the oil could have dissolved more gas. Below it the oil is saturated, a free gas saturation can grow, and both the oil formation volume factor and the solution gas-oil ratio decline.
It is read from a constant-composition expansion in a PVT cell. For a black oil it is the saturation pressure. A gas condensate has a dew point instead, where liquid drops out of the gas, and the two should not be swapped.