Molar solution is used when referring to the molarity of a solution, which expresses its concentration.
For example, a five molar solution of aqueous hydrochloric acid (written as "5M HCl (aq)") means there are 5 moles of HCl per liter of solution. If the solvent is not mentionned (such as "5M NaOH"), it is safe to assume that the solvent is water or the one most commonly used with that solute.
For work with aqueous solutions, concentrations expressed in terms of molarity are most useful when performing stoichiometric calculations since easily measured volumes correspond directly to moles of chemical substances involved.
