What is a relay switch?

I'm want to connect an amp and subs to my car but my stock head unit does not have a remote wire. I was thinking of hooking it up to any 12v wire I can find but I want to cut the wire in half and somehow put in an On/Off (----wire---switch---wire) (do they make does? where can I get one?) is that what a relay... show more I'm want to connect an amp and subs to my car but my stock head unit does not have a remote wire. I was thinking of hooking it up to any 12v wire I can find but I want to cut the wire in half and somehow put in an On/Off (----wire---switch---wire) (do they make does? where can I get one?) is that what a relay switch is?
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