Darüber sind die Gläser.
This one popped up when I was re-doing a past lesson, and I've never seen it before. It's telling me it means "The glasses are on top." but when I mouse over Darüber it says it means on them/over them? How does this word work?
Using "da" in front of a preposition is a common way of referring back to whatever you're talking about. It's the same in English, it's just that you stick two words together that don't always reside next to each other in English. And it works with other prepositions too: davon, daraus, dabei, damit, danach, dazu . . . virtually every preposition you can think of
Perhaps its the order of the words. Literally the sentence could be read as "Over them are the glasses" therefore implying the glasses are on top. Just a guess!
Darüber separates as da + über, lit. "above that". Such words will have diff meanings., which change according to the meaning of the sentence..
To generalize from your question, prepositions require nouns to follow them. This is the same in English - you can't say, "The glasses are over". In this case, we would rephrase it as "on top", while in German, a "da" is prefixed to the preposition to indicate a more absolute position. Not over, but on top. Not out, but outside. Not in, but inside. Etc.
salfordphil is right. e.g. if the glasses are over something, they are "darüber".