When the water molecules come together, as the water cools, they begin to interact closely, forming stable H-bonding. These structures begin to move about until all are maximized, forming a crystal. The crystal contains lots of space, more space than in the liquid state where all of the molecules are bumping in and around one another. As the crystal forms, the molecules actually move away from each other - expanding the amount of space they occupy. This is why the pipes in your house burst when the water freezes and also why the ice cubes in the ice cube tray have to be coerced out of the tray.
Normal substances contract when they convert from liquid to solid.
The solid is more tightly packed, more dense than the liquid. But, because the crystal form of water contains lots more empty space, it is less dense than the liquid form. There is less mass per volume. The liquid water is more dense than the solid water - which is why ice cubes float on water. Units of density are, for example, grams per milliliter, pounds per cubic foot, kilograms per liter.