A concentrated solution is one in which there is a large amount of substance present in a mixture. The degree of concentration is measured in moles.
An aqueous solution contains at least two substances: the solvent, water and a solute — the matter that will be dissolved in the water. The amount of solute that is dissolved in a solvent is the concentration. To keep track of the concentration, you can keep track of the mass. However, many find it easier to measure liquids using molarity, which is the number of moles of solute in a solution divided by the volume of the solution in liters.