Pharmacokinetics (abbreviated as PK in medical reading material) is basically the study of how a drug travels through the body. It's a "road map", if you will, of the "trip" drugs take through one's system.
By measuring things like the plasma concentration of the drug over a given time period, measuring them in both the same and in different subjects, many "maps" are created. Studying them provides a picture of how a given drug behaves in most who use or take it. Things like the life or halflife of a drug in the system, onset, peak, absorption, and duration all come from pharmacokinetic measurements. It's pharmacokinetics which allows a doctor to know how Drug X at Y mg dosage will work in a patient--when the drug begins working, when it works hardest, or peaks, and when the drug wanes and is leaving the system. This knowledge is why you are sometimes told to take medications more than once a day.
We tend to focus more on the pharmacokinetics of insulins, dealing with diabetes, but all drugs have pharmacokinetic profiles.