A ligand is a molecule or ion that forms dipole bonds to metal atoms.
In the Chemistry syllabus you only need to understand how water functions as a ligand. Certain ionic substances (such as copper sulphate) can exist in a hydrated form. This happens when the ionic substance is mixed with water: The ionic bonds between the substance are broken, leaving anions and cations floating in solution. Then, the positively charged hydrogen ends of the H2O molecules become attracted to the negatively charged anions, and vice versa for the oxygen ends and the cations.
The end result is that the water molecules surround and attach themselves to the molecules of the ionic substance with an ion-dipole bond. For example, the hydrated form of copper sulphate will be surrounded by 5 molecules of water. This process is known as water of crystallisation.