Molarity is the most widely used unit to express solute concentration in a solution. Molarity refers to moles of solutes per litre of solution.

The molarity of a solution can be calculated as follows:

A solution which contains 1 mole of solute per litre of solution is called 1 molar.

A mole of a substance contains 6.02 x 1023 (Avogadro constant) molecules of that substance. 

One mole is equal to the molecular weight of the substance in grams.

                                                     1 mole = molecular weight of substance

For example, 40 gram of NaOH (Molecular weight = 40) is equal to 1 mole of NaOH.

Mole is calculated by dividing the total amount of substance (weight in gram) by molecular weight of that substance.
So if a solution contains 40 gram (1 mole) of NaOH per litre it is said to be 1 molar NaOH solution.