Can someone explain the concept of seed phrase to me?

I know that when you purchase a wallet you get to set up a seed phrase, but what is it exactly?

Is it unique only to BTC blockchain? Or to which place does it register itself once has been set up? Is it like a personal ID? Shortly put, lots of questions.

My most confusing part is that a new device allows you to set up a seed phrase, but yet if the device is lost you can recover your funds with it. I'm lost on this. I used to think that each seed is individual to each blockchain (btc in this case) or is it like universal to all of them?