1. The problem is to understand the meaning and use of the unit "mM". 2. "mM" stands for millimolar, which is a unit of concentration used in chemistry and biology. 3. The prefix "milli" means one thousandth, so 1 mM = $10^{-3}$ M. 4. Molarity (M) is defined as moles of solute per liter of solution. 5. Therefore, 1 mM means there is 0.001 moles of solute in 1 liter of solution. 6. This unit is commonly used to express concentrations of substances in solutions, especially in biochemical contexts.