When you create a wallet, it shows you a list of 12 or 24 ordinary English words. This is your seed phrase, also called recovery phrase.
The master secret
The words are a human friendly way of writing one large random number. From that number, your wallet generates all your private keys and all your addresses, always in the same order.
The words come from a standard list of 2,048 words, so any compatible wallet can read them. If your phone breaks or your hardware wallet is lost, you type the words into a new wallet and everything comes back.
Stronger than it looks
Twelve words chosen from 2,048 give more combinations than anyone could ever try. Nobody can guess your phrase. They can only find it if you leave it somewhere.