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[1].
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[2]. 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[3][4]. 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[5][6][7], dealing with diabetes, but all drugs have pharmacokinetic profiles.