Not all solutions are created equal. Some are more concentrated than others. Molarity is one means of expressing how concentrated a solution is with solute. Molarity is defined as the number of moles of solute per liter of solution. Its units are mole/liter, sometimes abbreviated with a capital M. Knowledge of the moles of solute and the liters of solution allows one to calculate molarity by division.
 

The molarity of a solution is equal to the moles of solute per Liter of solution.
 
For two solutions with the same volume, the one that has the greatest number of moles of solute mixed into it will have the greatest molarity. The molarity is directly proportional to the number of moles. So if a solution has two times as many moles (but the same volume), then it will have two times the molarity. And if it has three times as many moles, then it will have three times the molarity.