The segments which show constant temperature means that a change in state is taking place.
The possible state changes in a cooling graph are condensation and freezing.
The possible state changes in a heatin graph are melting and boiling.
No Temperature Change = State Change
You can identify the portions which is undergoing state change by looking at which portion is experiencing no temperature change.
is the temperature at which a liquid changes into its gaseous state.
Melting point is the temperature at which a solid changes into its liquid state.