At the molecular level, ice exposed to heat will experience an increase in its temperature. In its solid form, ice is a crystal. Its molecules are relatively fixed, but they do still move if the ice is above absolute zero (0 degrees Kelvin, the temperature at which molecules are motionless).
Heat is a form of energy. Heat energy causes an increase in the temperature, or the average kinetic energy of the molecules of a substance. In other words, the molecules of the ice will vibrate more quickly. Molecules in a solid still move but are held closer together thanks to the attractive forces between them. As ice heats, its molecules will move farther apart.
Eventually, once ice reaches a particular temperature in normal Earth conditions (that is, typical atmospheric pressure), the ice will change phase into liquid form, water. This temperature, ice's melting point, is zero degrees Celsius or 32 degrees Fahrenheit. Liquid water no longer has the crystalline structure of ice, and its molecules move relatively freely.