The formula unit is an empirical formula of a compound which gives the simplest whole number ratio of atoms of the element present in the compound. For e.g. the formula unit of glucose is CH2O, while the actual formula for glucose is C6H12O6.
The formula unit mass is the equal to the sum of atomic masses of all the atoms in the empirical formula of the compound. For e.g. the formula mass of CH2O is 30.