Prior knowledge: You should understand the difference between a mixture and a pure liquid. You should understand what ions are, and the idea of a giant ionic lattice structure.
A solution is a mixture formed when a solid substance dissolves in a pure liquid. Both substances are still present in the mixture, and only the physical state of the substance that was solid has changed.
We call the pure liquid that we are going to dissolve something in a solvent. Water is a particularly good solvent, but other substances such as alcohols are good solvents too.
We call the solid substance that dissolves in the solvent a solute. Many substances with giant ionic structures dissolve in water (e.g. sodium chloride). The process of dissolving requires the ionic bonds that are holding the ions together in the lattice to be broken. As a result of this, once dissolved the ions are completely separate from one another and are all free to move around within the solution. We could show the dissolving of sodium chloride as:
NaCl(s) → Na+(aq) + Cl–(aq)
We can keep adding more and more solute to a given volume of solvent. Eventually no more solute will be able to dissolve. The solid we add will simply sit on the bottom of the beaker no matter how much we stir it. The solution is said to be saturated. The amount that we can dissolve depends on the temperature of the solvent, so if we warm up a saturated solution, we will be able to dissolve some more solute. If we cool down a saturated solution, it will not be able to contain as much dissolved solute, so crystals of the solid solute will form.