Energy must be added to separate the water molecules as ice melts.
Ice is solid, and when it melts it becomes liquid. When you add heat to ice, it melts. This is basically the process of adding energy. The first link I provided below has a very good explanation. As you can see, when water is a solid the molecules don’t move much. When you add energy, or heat, the molecules begin to move. This is the process of ice melting. Once the ice melts, it is no longer a solid and becomes a liquid.