Millimolar (mM) is just a way to express concentration. It means one-thousandth of a molar (M), so 1 mM = 0.001 M. I use it a lot in lab work when I need very dilute solutions.
Millimolar (\(mM\)) is a unit that measures the concentration of a substance in a solution and is equal to one thousandth of a mole per liter 10^-3 mol/L. It shows the molarity of the solution, which means the quantity of a solute in the solvent.