efficacy
If affinity is whether a key fits the lock, efficacy is how far it can turn the bolt once inside. It is the size of the biological effect a drug can generate after it has bound to its receptor.
Efficacy is what separates an agonist from an antagonist: an agonist has high efficacy and produces a strong response, a partial agonist has intermediate efficacy and a smaller ceiling, and an antagonist has zero efficacy — it binds but cannot activate. In its strict sense, efficacy reflects how effectively the bound drug couples the receptor to its downstream signalling machinery.
Clinically, the word efficacy is also used more loosely to mean how well a treatment works under ideal trial conditions. In pharmacodynamics, though, it is best kept distinct from potency: potency is about the dose required, while efficacy is about the maximal achievable effect (the Emax) once you have given enough drug.
Full opioid agonists like fentanyl have high efficacy and can suppress breathing completely, whereas the partial agonist buprenorphine has a lower efficacy ceiling.
The maximal effect a drug can drive.
Higher efficacy does not mean a drug is better. Sometimes a partial agonist with lower efficacy is safer because its built-in ceiling limits dangerous over-stimulation.