Binding affinity, efficacy, and potency are related, but they are measuring different things. Each of them is more important in particular contexts.
Binding affinity is a measure of how strongly a compound binds to an enzyme. This is measured directly by allowing a purified compound to bind to a purified enzyme. It tells you nothing about what effect the compound binding to the enzyme might have, or to how the compound gets to the enzyme in the first place. If the compound is an antagonist and it bind in the active site, then you can usually work out a correlation between the binding affinity and enzyme inhibition, but if the compound is an agonist or acts on some part of the enzyme other than the active site, the mere strength of its binding does not necessarily relate to the activity of the enzyme at all.
Efficacy is a measure of the effect on some biological property. For efficacy to be measured, you must have a working biological system in place, even if it is as simple as a transfected cell with a reporter gene. For a compound to show efficacy, it must affect the biological system, not just interact with a purified enzyme, and cells have all sorts of mechanisms and structures that might limit these possibilities. For example, if your enzyme only works inside the cell and your inhibitor can't pass through the cell membrane, it doesn't matter how strong your binding affinity is, you won't show any efficacy.
Potency is defined in a couple of ways, but always in reference to some sort of activity. You can have potency in an in vitro assay, where you are measuring the activity of an enzyme in the presence and absence of an inhibitor. A potent compound in this assay would be one in which a small amount of compound would show a large change in the measured activity of the enzyme. You can also have cell potency or in vivo potency, in which a small amount of compound will show a large change in a biological measure in the cell or the test animal respectively. It is quite common, and very exasperating, for a remarkably potent compound in vitro to show very low potency in the cell, and even more annoying for a compound to show high potency in both in vitro and cell assays but low potency in the animal. This is produced either by mechanisms that keep the test compound from reaching the enzyme that it is to inhibit, or to other biological mechanisms that counter the effect of inhibiting the enzyme.
That is a very, very broad and shallow overview of the topic. I would be happy to elaborate on any areas that you feel would be helpful.