How do molecules of solid move?
Not counting how they all move together as the object moves;
each molecule moves around its own location (often said to vibrate) at a speed that depends on the temperature of the object.
If the temp gets high enough, then the molecule(s) can break loose of the bonds that hold them in place ... either liquefying or evaporating.
At absolute zero they don't move, theoretically (for that temp has never been reached).
melting. Yep, it sure is melting. But also, when a solid becomes a liquid, it basically is heat being applied to the solid, and causing the molecules of the solid to move around, and when the molecules move around, it is not called a solid anymore, but called a liquid, because the molecules are not compact. :D -Dain