efficacy

If potency asks 'how little do you need?', efficacy asks 'how good is the result when you give enough?' Think of two keys that both fit a lock: one turns it all the way and opens the door, the other only half-turns and the door stays shut. Both keys 'bind' the lock, but they differ in how well they make the lock do its job. Efficacy is that capacity to actually produce the biological effect.

More precisely, efficacy describes the maximal effect a drug can elicit once it is bound, reflecting how effectively binding is converted into a downstream signal. A drug that can drive a receptor to its full response has high efficacy (a full agonist); one that can only ever drive a partial response has lower efficacy (a partial agonist); and a pure blocker that binds without activating has zero efficacy (an antagonist).

Efficacy and potency are independent axes. You can have a highly potent drug with modest efficacy, or a low-potency drug with full efficacy. Clinically, efficacy often matters more for the ceiling of benefit, while potency mostly sets the dose. Note that 'efficacy' is sometimes used loosely in clinical trials to mean 'does the drug help patients', which blends the receptor-level idea above with absorption, distribution and many other factors — the pharmacological meaning here is the narrower, mechanistic one.