Serum blood level describes the amount of a given medication present in your blood at the time of testing. Blood serum is the liquid part of the blood that contains no clotting factors or blood cells.
Some medications used to treat bipolar disorder require serum blood level testing. People taking lithium in particular must be tested regularly, as there is a narrow "therapeutic window" for lithium. That means that the difference between a therapeutic level and toxic level can be small in some individuals.
Plurals: Serums (as above), Sera: SEER-uh