Positive Feedback, as it is applied to Mafia, is a phenomenon where a faction that starts to win gets stronger, thus making it more likely to continue winning.
Examples of this are very easy to find. As the Mafia eludes the Town's eliminations, they will eventually start to hold more and more of the voting power of the Town; by ELo, it is both difficult and unlikely for the Town to pull itself together and eliminate scum unless the scum wish it so. On the other hand, a Mafioso whose partners have been picked off must not only survive more eliminations, but also has little defense against the Town's power roles.
While this is not a particularly desirable trait, it is an inherent part of Mafia's structure. As a response, some moderators have sought to innovate the game by finding ways to reduce this effect. The Enabler and Suicidal pages have some examples of how this has been done. Another means has been to structure the game mechanics so that factions that start to win by a large margin have members removed from the game (at a rate slower than the normal rate of death so as to maintain but diminish their lead, of course).
For the opposite, see Negative Feedback.