Saturated liquid is a state of a substance, typically a liquid, where it exists at its boiling point temperature under conditions where any additional heat added to the substance will cause it to change phase into a vapor (gas) at constant temperature and pressure.
In the context of water, for example, saturated liquid water is water that is at its boiling point temperature (100 degrees Celsius at standard atmospheric pressure). At this point, the water is in equilibrium with its vapor phase. If you continue to add heat to saturated liquid water, it will absorb the energy and transition into a vapor phase, while the temperature remains constant until all the liquid is vaporized.
