The 2 level portion in the graph represents the changing of state.
Every substance has internal energy, which includes kinetic energy and potential energy. Kinetic energy means the temperature, and the potential energy means the bondings or attractions.
When a substance is heated up, they first increase their temperature as the same state. For example, the ice starts at - 5 °C, they won't start melting immediately as they're not at their melting point yet. Instead, they first absorb heat and increase their temperature to 0°C. This is same for when water increase their temperature until their boiling point. Their kinetic energy is increasing, but potential energy is unchanged as they stay at the same state. That's why the slope is increasing.
However, but once the ice or water has reached their melting or boiling point, they have to go through a state change. During that time, they do not increase their temperature (K.E. unchanged) . Instead, the heat they absorbed is used to increase the potential energy to break the bonds and turn into another state. The heat absorbed is called latent heat.
This explains why in the graph, there's 2 level portions, as the substance is going through a state change and increasing their potential energy instead of their kinetic energy (temperature)
