Fish Mooney's plant was a plant owned by Fish Mooney.
Jim Gordon and Harvey Bullock are taken to the plant after Jim interrogates Fish Mooney. It is her 'disposal site' of choice, a fact known to the cops, as Bullock points out when he and his partner are set for the chop. Don Falcone, however, steps in and prevents this particular execution, although he causes several other deaths in the process and leaves Butch Gilzean wondering if he himself will see another dawn.
Fish's former umbrella boy Oswald Cobblepot was also being held at the plant after he had snitched on her to the police. Falcone went to see him, and Oswald told him that Fish was planning on taking him down, and convinced Falcone to let Jim "kill" him, as he knows Jim will more than likely spare his life and he will return to Gotham under another identity and be his snitch. Falcone takes Oswald up on his offer, which would be executed perfectly as Oswald had planned.