M&M is the shorthand for marines and medics, one of the most potent tactics in Starcraft available to Terran players, often combined with the tank push to form a dangerous attack force. When firebats are added to the party, the term becomes MMF, losing the little sign for coherency's sake.
A player using M&M often starts the game with two barracks and a refinery, quickly obtaining an academy to be able to train medics. Early game battles usually are initiated when there is a hotkey of marines available with three or four medics to accompany them, often with the two potent upgrades available at the academy. Later in the game, a Terran player is pumping these infantry units out of upwards of six barracks, attacking in swarms with more than fifty of the soldiers and a sizable escort of medics. This strategy is most potent against Zerg players, as most of their units are either torn apart by the marines before they can close to attack range or simply do little damage to the soldiers. Versus Terran opponents, a player using M&M can often take out an opponent who is taking the time to go heavy metal, as the infantry force comes out quicker than the other.
Protoss forces, on the other hand, are usually quite capable of resisting an M&M attack. Their ground units usually possess armor ratings that put a damper on the marines' attacks, and usually can also micro away the blunt of a focus-fire Terran's attack. Their most potent weapons, though, are High Templar armed with Psionic Storm, the Dark Templar, capable of slaughtering an unsuspecting marine in a single blow, and the Reaver, a unit that can take out four or five marines with a single blow. If the game lasts long enough, Zerg players can tech towards Guardians and ultralisks- both of which, if properly used, destroy armies of M&M without an afterthought. Terran players field M&M of their own- and siege tanks put even more hurt on them.