When you heat water in a pot and it reaches 100°C, a phase change occurs from liquid to gas as the water starts boiling. However, once the water reaches its boiling point, the energy being supplied to the system is primarily used to convert the liquid water into steam (water vapor) rather than increasing the temperature further.
The temperature of a boiling liquid remains constant because the heat energy being added to the system is absorbed by the water molecules as latent heat of vaporization. This latent heat is required to break the intermolecular bonds holding the water molecules together in the liquid phase and convert them into a gaseous phase. As a result, even though the water is continuing to receive heat, its temperature remains steady at 100°C until all of the liquid has been converted into vapor.
The energy absorbed during this phase change does not contribute to an increase in temperature until all the water has vaporized. Once all the water is in the gaseous state, further heat input will increase the temperature of the steam. This phenomenon explains why the temperature of the pot does not stay at 100°C indefinitely, even when the water is still in the process of vaporization.