In GCSE, you probably did an experiment where you took some ice, heated it steadily with an electric heater and measured the temperature as time passed. You should have got these results:
Even though you heated constantly, the temperature only rose sometimes. So where was the heat going when the temperature wasn't rising?
Answer: it was being used to change the state of the water.
Remember in the energy topic we said that the energy of an atom was made up of two components - Ek and Ep. Well:
The Ek component relates to the temperature of the substance
The Ep component relates to the state of the substance.
So during times of increase of temperature (e.g. a, c and e from the graph) the heat energy supplied is going into the Ek component of the internal energy.
And during changes of state (e.g. b and d) it's going to Ep. Because Ek isn't changing during these times, the temperature remains the same.