To understand this take some ice cubes in a beaker and note the temperature of ice. Slowly heat it. It is observed that there is no change in temperature as long as there is ice in the beaker.
In this process, even though the heat is continously supplied, but the temperature does not change. The heat supplied is used in changing the state from solid to liquid (ice → water).
The temperature remains 0°C until the ice melts. At this point, further heat transfer will produce a temperature increase of liquid phase till it reaches 100°C, the boiling point of water. It remains at 100°C till all the water change into gas (steam).
