Meq is the short form of milliequivalent, while Mmol is the short form of millimole. Both are units of measurement for different kinds of substances. The only similarity between the two is that they are both used for measuring substances, but their differences lie in the exact substances which they can be used for. For instance, milliequivalent is used as a unit of measurement for a particular substance, and the same thing is applicable to millimole.
Meq refers to one-thousandth of an equivalent. It is used most times as the unit of measurement for chemicals, especially electrolytes. Electrolytes are substances (in solution or molten form) which easily ionize when electric charges are made to move in them. Millimole, on the other hand, is an SI unit, equivalent to one-thousandth of a mole. While Meq is mostly used as the unit of measurement for electrolytes, millimole can be used for things like an electron, atom, molecule, etc.