When water is in liquid form its molecules are free to move around.
Water molecules are packed reasonably close together. However when water freezes its molecules take up a hexagonal lattice (repeating structure) which has space in the middle of it.
This is largely due to hydrogen bonding between water molecules (complicated).
As a result water molecules in ice aren't packed as closely together as they are in liquid water so the density of ice is lower than that of liquid water.
Hope that helps. I doubt you need to know about hydrogen bonding.
