What is the 48v relay for?

[4wilsons]

Ok, let me try and word this so you guys understand. I recently got the brake light switch that hooks to rod under the floor. It came with a 48v 4 pole relay.
What exactly is the relay used for? I know what relays are used for but not sure on this one. I'm not using new wiring harnesses. I was told that the brakes won't work without, but they do. If it's a 48v relay, does that mean that the whole 48v goes into it? Wouldn't that also mean that 48v comes out when switched? Wouldn't the 48v blow the lights and switches out?

Thanks in advance

Sorry, it's a 09 precedent 4x12