Pharmacokinetics (PK), in general, describes how the body affects a drug once it is administered until it is eliminated. Population PK studies a drug's PK and its associated sources of variability among individuals of the target patient population. Sources of variability include but are not limited to demographics (body weight, height, gender, etc.), pathophysiology, as well as concomitant medication.
Population PK seeks to identify measurable sources of variability in drug concentrations and to relate these to the respective pharmacokinetic parameters. If the changes are deemed clinically significant, the therapeutic regimen can be adjusted appropriately.