A storeroom was a place where supplies of various commodities were kept.
In 2367, Barash, masquerading as "Ethan", helped Riker escape through a storeroom the Romulans had forgotten about. (TNG: "Future Imperfect")
In 2370, Rom assured Quark that he could open a pulsatel lockseal in 10 seconds, as he had sometimes had to unlock the one on Quark's storeroom to serve customers. (DS9: "Necessary Evil")
In 2371, Quark explained to Jadzia Dax that he'd had three cases of Kanar in his storeroom since the Occupation. (DS9: "Destiny")
Later that year, he asked Nog to fetch Rom from the storeroom to man the bar in his place, as he had a senior staff meeting to attend. (DS9: "Facets")
Also in 2371, Constable Odo found 27 Cardassian voles in Quark's storeroom. Quark claimed that he and Morn were catching them and counting them rather than numbering them for a fight as Odo asserted. (DS9: "Through the Looking Glass")