What it is
When you create a wallet, the app generates a sequence of 12 (or 24) English words, following a standard called BIP39. From those words, every key in the wallet can be recreated, in any compatible app.
In practice: whoever has your words has your money. They're worth more than your phone and more than any password.
How to store it
The goal is that nobody else ever sees the words and that you never lose them.
- Write it on paper, by hand, in the right order, double-checking every word.
- Keep it somewhere safe and dry; for larger amounts, consider two copies in different places.
- There are metal plates made for engraving the words, which survive fire and water.
- Don't take a photo, and don't save it in the cloud, email, a notes app or a screenshot: phones and accounts get hacked.
- Don't type the words into websites. They should only ever be typed into the wallet app itself, when restoring it.
Anyone who asks for your words is a scammer
No support team, company, exchange or "expert" needs your words to help you. A request for your recovery phrase, through any channel, is always an attempt to steal from you.