The Critical Banker perk allows you to save up an increasing number of Critical Hits, to be used when they're needed most.”— Fallout 4 loading screen hints
With the first rank of this perk, you have the opportunity to store a critical hit in V.A.T.S. for to use at a later time. The second rank allows you to store a second critical hit for use in the future. The third rank allows you to store a third critical hit for a later chance, and gives a chance to store an additional critical hit whenever a critical hit is stored.
With Far Harbor, the perk gains a fourth rank, allowing for a fourth critical hit to be stored.