Here's how it breaks down:

* milli- (m): A prefix meaning one-thousandth (1/1000).

* molar (M): A unit of concentration that represents the number of moles of a substance per liter of solution.

Therefore, 1 mM (millimolar) = 1/1000 M (molar).

In biology, mM is commonly used to express the concentration of:

* Solutes: Substances dissolved in a solution, such as sugars, salts, or proteins.

* Chemicals: Like enzymes, hormones, or drugs.

For example, a solution with a concentration of 10 mM glucose contains 10 millimoles of glucose per liter of solution.