A seed phrase is the master key for a wallet and it can be either 12 or 24 words long. With a wallet's seed phrase you can recover the wallet on any device, and have full access to the crypto that it holds.
The seed phrase for a wallet cannot be changed, and it is permanently tied to the wallet addresses that are derived from it. So it is very important to not share the seed phrase with anyone else, and keep it securely backed up at all times!
