What is meaning of 0.6m of phosphoric acid?

Answer 1

this means 0.6gm of phosphoric acid in 1000gm of water --------------------------------------- (the previous answer describes a mass percent) It means a 0.6 molal phosphoric acid (i.e. 0.6 mol hydrogen phosphate in 1 kg of water, which is roughly equivalent to 0.6 M phosphoric acid), though molality (mol/kg) is rarely used to denote concentration of acids. More common units are molarity M (mol/L) or normality n (mol/L of hydrogen ion generated).

Answer 2

0.6m of phosphoric acid means that there are 0.6 moles of phosphoric acid dissolved in 1 liter of solution. It is a measure of the concentration of phosphoric acid in the solution.