In real world biology, serum is the clear yellowish fluid left over from blood when you separate out the blood cells, normally by spinning in a centrifuge. It can still contain antibodies among other things, but no red cells.
In recent decades the word has also been misappropriated by elements of the cosmetics industry for use in describing some of their ...
In addition to the other two answers, serum could also be a blind idiot or false friend translation. In German, serum has mutated in common use from the scientific definition via antiserum to become medication in a more general sense.
Compare the use of truth serum in English.
Apart from being part of the blood, "Serum" might also mean medicine that is made from blood.
E.g. Merriam-Webster names as a possible meaning:
medical : serum from an animal's blood that can be added to a person's
blood to prevent or cure disease
From there it's speculation, but it is easy to imagine how this would come to be used as a synonym for ...