The point where you have enough of one unit that it beats things it would normally lose to. The term is used most with Archers and Crossbowmen: ten Crossbowmen die to a few Knights, but forty Crossbowmen with upgrades shoot Knights down before they arrive.
Reaching critical mass takes time and resources, so armies built on it are weakest early in their life. Splash damage from Mangonels and Onagers is the usual answer to a large group of archers.