Seed phrase (also known as mnemonic phrase) is used for convenient wallet backup. It consists of 12 – 24 words. For example:
act pull only shy inner proof spot power stem rate hello vote
It is much easier to write down easy to read seed phrase rather than writing down unreadable private keys.
Seed phrase, supported by many wallets, is used to generate private keys. Thus, seed phrase is as important as
private keys and must be kept secret. If bitcoin wallet is lost, seed-phrase is used to recover the wallet.