a lampstand: the raised stand that holds an oil lamp so its light reaches the whole room
A lampstand is not the lamp but the thing that holds it. In the ancient world a lamp (λύχνος) was a small clay or bronze dish of olive oil with a wick, and the lampstand was the raised stand, shelf, or bracket that lifted that little lamp so its light could fill the room. The two words sit side by side in the same sentence: people light a lamp and then set it on the lampstand. This was an ordinary household object, common enough that Greek shopping lists and temple inventories mention it for centuries before the New Testament.
The same plain word carries a sacred weight, because the Greek Old Testament used it for the golden lampstand of Israel's tabernacle, the seven-branched menorah (מנורה). So when the letter to the Hebrews lists the furniture of the holy place, this is the word in view. For a reader who knew the Greek Old Testament, a lampstand in a place of worship pointed straight to that golden menorah and the presence of God it stood for.
The book of Revelation gathers up both threads. John sees seven golden lampstands and is told plainly that they are the seven churches, with Christ walking among them as a priest tends the lamps. A church, in other words, is pictured as a lampstand: its job is simply to hold up the light. The warning to one church, that its lampstand could be removed, is a warning that a community can lose that role.