The amount of a substance (solute) in a solution
A large organ in the upper abdomen which manufactures, stores and breaks down substances as required by the body
How well a drug works and how long it acts for depends on how much there is in the blood stream (the concentration in the blood). This concentration varies with time depending on how quickly it gets from the stomach and how quickly your body gets rid of it. It is just like trying to fill a bath with a leaking plug - as some goes in part of it runs out.
Imagine what happens to the level of water in the bath if you gently pour one whole bucket of water in. The level goes up quickly but then starts to drop as the water leaks out. It is the same with levels of drug in your blood. Just think of the bucket of water being a tablet of drug.
Different medicines have different blood level profiles. These profiles depend upon their relative rates of absorption from the stomach and loss through being broken down by the kidneys and liver.