affinity
Affinity is how 'sticky' a drug is for its target — how strongly the two cling together once they meet. A high-affinity ligand grips its receptor and stays bound even at very low concentrations; a low-affinity one keeps slipping off and needs much higher concentrations to stay attached.
Affinity is set by the molecular interactions at the binding interface: hydrogen bonds, salt bridges, hydrophobic contacts, π-stacking and van der Waals forces, all summed into a binding free energy. It is most commonly quantified by the dissociation constant Kd (or, for inhibitors, Ki): a smaller value means tighter binding, because less drug is needed to occupy the target.
Affinity should not be confused with efficacy. Affinity only describes how well a molecule binds — not what it does once bound. A high-affinity ligand might be a powerful agonist, a silent antagonist, or anything between; binding strength and biological action are separate axes that medicinal chemists tune independently.
A drug with a Kd of 1 nM has roughly a thousand-fold higher affinity than one with a Kd of 1 µM, and will occupy the same target at a far lower dose.
Lower Kd means tighter binding and higher affinity.
Affinity is a snapshot of binding strength at equilibrium; it says nothing about how fast binding happens. Two ligands with identical Kd can have very different on- and off-rates, and hence different residence times.