PG67A/W is a special, Progenitor-based serum used to keep the virus within Albert Wesker's body stable and stop him from mutating. The "PG" may be an acronym for "Progenitor". The "A/W" itself suggests that it was specifically developed for Wesker, and was administered to him regularly by Excella Gionne. The dosage given must be extremely precise and taken at certain times, otherwise it can act as a poison.
During the second fight with Wesker the player's mission is to inject the serum on him, causing an overdose that would ultimately kill him.