When ice melts 80 calories of heat are absorbed. A calorie is the amount of energy needed to raise the temperature of 1 gram of water one degree centigrade. Imagine an experiment in which you put a thermometer on a block of ice and increase the temperature to 50 degrees centigrade. The temperature of the ice will rise until a temperature of 0 degrees is reached. The temperature then remains constant until the ice melts to water.
Heat is absorbed until all the ice is melted.
Then, and only then will the temperature rise. Think of water as the system and everything else are the surroundings. A plot of temperature versus pressure provides a useful framework in which to examine some of the characteristics of these three phases. Note the direction of increase of the temperature and pressure axes. Think about the following questions.