Q: What is a secondary dominant?
A: don't worry about it. those names are going to confuse the hell out of you. i don't recommend calling them that, and i think most people do not call them that. but anyway, you know how in songs, like in the key of C, you'll get G7 then C. like coming home, right. well, sometimes, you'll hear D7 before the G7, then C. try it. D7, G7, C. i think that's what they mean when someone says the first thing you said. but like i said, i don't call it that and don't know anyone who does, because the names don't really make any sense. well, the name 'secondary' does i suppose, but 'dominant' i think will throw people off. it's not dominating anything. it actually seems more like it's the final chord that dominates the whole thing, if you ask me