When you create a wallet, it shows you 12 or 24 words. This seed phrase is the master backup: from it, every key and address in the wallet can be rebuilt.
How it works
The standard, BIP39, turns random data (128 to 256 bits) plus a checksum into words from a fixed list of 2,048. The checksum lets wallets catch typos. Anyone who has the words can recreate your wallet on any compatible app, anywhere.
Protect it
- Write it down on paper or metal, in order. Check spelling.
- Never photograph it, type it into a website, or store it in the cloud or email.
- Keep it somewhere safe from fire, water and curious eyes.
- Never share it. No real support agent will ask for it. See phishing.
Optional passphrase
BIP39 allows an extra passphrase (sometimes called the "25th word"). It creates a completely different wallet. It adds protection, but if you forget it, the coins are gone.
Takeaway
Your seed phrase is your bitcoin. Losing it means losing access; sharing it means losing the coins.