What is a seed phrase?

A seed phrase is a list of words that stands in for a large random number. From that number a wallet derives its keys. Write the words down and you can rebuild the same keys in any wallet that follows the same standard. Lose the words, and the keys, and the coins are gone.

Twelve or twenty-four

BIP39 is the list most wallets use: 2,048 English words. Twelve words carry 128 bits of entropy plus a checksum. Twenty-four words carry 256 bits plus a checksum. The checksum is why a wallet can often tell you a word was written down wrong. Often, not always: some mistakes still produce a valid phrase for a different wallet.

The words are not a password for a website. They are the secret. Anyone who has them can derive the keys and take the coins, from anywhere, without your device.

The optional passphrase

Some wallets add a passphrase, sometimes called a 25th word. It is not one of the 12 or 24. The same word list with a different passphrase produces a different wallet. If you set one, it has to be backed up too. Forget it and the words alone restore an empty wallet.

Never type the words into a website, a computer or a phone app (the only exception is the wallet you are restoring into), say them on a call, or photograph them. A backup is paper or metal, in a place a camera does not see. The guide on backing up walks through the order of operations.