A saturated solution is one in which any additional solute added to the solution is no longer dissolved. Solutions are combinations of solvents – most commonly liquids - and solutes, which are typically solids.
Each given solution has a point at which any more solute added does not dissolve. A prime example of this is demonstrated by adding sugar or salt to a glass of water. There is a point when any more of the solid added to the liquid results in the solid remaining in the bottom of the glass. This is, however, dependent upon temperature; most solutions show an increased saturation point in response to increased heat for a given pressure. In the most basic terms, an increase in heat results in more solute dissolving within the solvent.
Solutes are also commonly gases, as with the oxygen in water. Even with water, there is a point at which no more oxygen is dissolved. This is when the additional oxygen introduced into the solution is released as bubbles. Most gas solutions respond to heat and pressure in the same manner as solids, with warmer air absorbing more water vapor than cooler air, as is the case in one common example.