Molar mass is the mass of one mole of a substance. It is usually expressed in grams per mole (g/mol).

Calculating Molar Mass: 

To calculate the molar mass of a substance, you need to add up the atomic masses of all the atoms in a molecule. For example, to find the molar mass of water (H2O), you would add the atomic masses of two hydrogen atoms (2 x 1.008 g/mol) and one oxygen atom (1 x 15.999 g/mol), which gives a molar mass of 18.015 g/mol.

FYI - 

Molar Mass and Molecular Weight: Molar mass and molecular weight are often used interchangeably, but they are not exactly the same thing. Molecular weight is the sum of the atomic weights of all the atoms in a molecule, whereas molar mass is the mass of one mole of that molecule.

Molar Mass and Avogadro's Number: Avogadro's number is the number of particles in one mole of a substance (6.02 x 10^23). The molar mass of a substance is directly related to Avogadro's number, as one mole of any substance will always have a mass equal to its molar mass.