An unsaturated solution is one that is able to have solvents added or removed. Unsaturated solutions always have fewer particles, or solutes, than solvents.
A solution is defined as having a solvent along with particles. The solvent that is present in a solution is generally always liquid, but can be made of gas. The particles that are in the solution make up the various chemical differences of the solution. Solute concentration is what dictates the amount of particles that are present in a solution. The concentration of the particles can change depending on the situation and can be affected by different factors, including temperature. Concentration can change when more particles are dissolved within the solution.
Iced tea with sugar is a simple example of an unsaturated solution. When sugar is added to iced tea, the particles of the sugar generally dissolve giving the tea a sweeter taste. When large amounts of sugar are added to the tea solution, the concentration of sugar and tea changes. Sugar that is added to the tea to an extent that there is no more room for more sugar to dissolve will cause the solution to become unsaturated. The particles of sugar will no longer dissolve and will reduce the amount of solvent, tea, that is in the solution.