One way to picture it: a plain building stacked to the roof with one kind of thing, kept there until somebody needs it. The word is built exactly that way — store plus house — so the image is right on the surface. Modern English rarely uses it for a real building any more; it spends most of its time saying that something holds as much as one of those rooms would.
From the word’s history
The packet gives the compound plainly: storehouse is store plus house, with store coming into Middle English through Anglo-Norman from Latin instaurare. Nothing here needs interpreting; the parts still say what the word means.
資料把這個複合字拆得很清楚：storehouse 就是 store 加 house。store 經由盎格魯諾曼語進入中古英文，源頭是拉丁文 īnstaurāre。這個字不需要額外解讀——兩個部分加起來，就是它今天的意思。
Wiktionary entry supplied in this packet (source_ref kaikki:3f6f69a190036ae4:972974175): 'From store + house', with store traced through Anglo-Norman from Latin instaurare.
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