A seed is usually a group of words 12 or 24 words
; these for most systems that if you lose access to that system meaning usually on a hardware wallet
You will get a phrase. A seed phrase that’ll be 12 to 24 randomly generated words that you write and store somewhere safe.
This way if you lose access to that device and where your keys are also stored you can then find a new device or usually go on their web version of their wallet, insert your seed phrase and be able to recover your account.
So it’s a backup...
It’s a backup just in case you lose your private keys or you lose your hardware.
They have your private keys
in them the seed allows you to gain access again to your wallet to your funds. Crypto simple as that.