Ice-cream melts because it is heated by the objects and air touching it and its melting point is much lower than room temperature. Heat is the transference of thermal energy. Thermal energy is the average amount of movement, or kinetic energy, of the particles within an object.
The greater the thermal energy of an object, the hotter it is. When an object with lesser thermal energy comes in contact with an object of greater thermal energy, such as when ice-cream comes in contact with room-temperature air, thermal energy is transferred from the object with more thermal energy, the air, to the object with less thermal energy, the ice-cream, until the energy is equal. However, the amount of energy required to raise the temperature of an object, known as specific heat, differs between objects. This is why some objects, such as many metals, feel hotter or colder than the objects around them even though they possess the same amount of thermal energy.