The molecules absorb heat and acquire more kinetic energy.
Explanation:
In a solid, the solids only vibrate about their mean positions but do not translate. When energy is supplied to the molecule in the form of heat, the molecules vibrate faster. Eventually, they acquire sufficient energy to leave their mean positions and translate. Hence the solid crystal collapses.
When ice is heated, water molecules acquire sufficient kinetic energy to translate. The intermolecular bonds are gradually broken in the solid framework as heat is absorbed. The heat required for this is known as the latent heat of fusion.
The temperature remains constant until phase transition is over, then temperature rise resumes.
