In an unsaturated solution, the amount of dissolved material is below the maximum limit. If you add more solute to this liquid, the new solid will dissolve completely. The solute particles break apart and mix into the solvent without settling at the bottom.
Temperature plays a big role in this dissolving process. Heating a liquid usually increases how much solid it can hold. This means a saturated solution can often become unsaturated if you simply warm it up.
Pressure also matters a lot when you dissolve gases in liquids. Increasing the pressure allows a liquid to hold more gas. Chemists compare unsaturated mixtures to two other types of solutions.
Saturated solutions hold the absolute maximum amount of dissolved material. If you add more solid to a saturated liquid, it just sinks to the bottom. Supersaturated solutions are carefully forced to hold more solute than normal.
They are very unstable and will quickly drop solid crystals if they are disturbed. Unsaturated solutions are the most common type of chemical mixture we see every day. Liquids like apple juice, ocean water, and plain tea are all unsaturated solutions.