A cup of tea is made up of the solvent, water, and a number of solutes: the tea, milk and perhaps sugar. A highly saturated solution produces strong, sweet tea. A poorly saturated solution produces weak tea.
Tap water is rarely pure water; it usually contains invisible traces of other substances. This makes it a solution.
A solution is a liquid that has a solid dissolved within it.
When a solid dissolves, its molecules separate and mix completely with the molecules of the liquid.
The liquid in a solution is called the solvent.
The solid dissolved in a solution is the solute.
The more of a solid that dissolves, the stronger the solution becomes until at last it is saturated and no more will dissolve. There is literally no more room in the liquid.
If a saturated solution is heated the liquid expands, making room for more solute to dissolve.
If a saturated solution cools or is left to evaporate, there is less room for solute, so the solute is precipitated (comes out of the solution).
Precipitated solute molecules often link together to form solid crystals.