What is an unsaturated solution? How could I prepare one?

Solutions are made by mixing a solute (usually a solid. Think of salt as being a solute) with a solvent (usually a liquid. Think of water being a solute). A "saturated solution" is a solution of a solute in a solvent where the maximum possible amount of solute has been dissolved. An "unsaturated solution" is, therefore, a solution where less than the maximum possible amount of solute has been dissolved.

You should be able to figure out how to make an unsaturated solution.