A supply depot was a space station that functioned as a storage installation. They could also act as outposts for starships to refuel.
In 2370, Calvin Hudson asked Benjamin Sisko for his permission to use Deep Space 9 as a supply depot for the Maquis, to which Sisko immediately refused. (DS9: "The Maquis, Part II") Later that year, the Maquis reprogrammed the Cardassian weapon ATR-4107 to destroy a fuel depot on Aschelan V. (VOY: "Dreadnought")
During the Dominion War, the Dominion had several supply depots in the Borias Cluster. General Martok planned to attack them in 2375. (DS9: "Once More Unto the Breach")