The Rakghoul serum was a special antidote for the rakgoul disease. It was designed by Republic engineers on Taris and stolen by the Sith when they took over. Revan finds the serum on the body of a Sith soldier in the Undercity.
In the Light Side choice, you can either give the serum to an infected Republic soldier (who later gets killed by rakghouls), or give it to the infected Undercity villagers. Or you could give it to Zelka Forn, who will offer you a few credits and medpacs. If you want to be even more Light Side, decline his offer and just give him the serum.
In the Dark Side choice, you give the serum to Zax in the Bounty Office. He offers you 1000 credits for it, but if you have a high Persuade, you can convince him to give you 1500 credits.