Short answer
A seed phrase is a list of 12 or 24 ordinary words that encodes your wallet's private keys. Anyone who has it can take everything in the wallet.
Wallets generate a seed phrase (also called a recovery phrase or mnemonic) when they are created. From those words, the software can rebuild every key and address the wallet uses, which is why typing the phrase into a new app restores the same wallet.
Treat it like the master key to a safe. Write it down on paper or stamp it into metal, store it somewhere private, and never type it into a website, a chat or a form. No legitimate support team will ever ask for it.
Some newer apps, DYOR included, let you start without writing down a phrase by tying the wallet to an email, Google or Apple sign-in. You can still export the key later and keep an offline backup, which we recommend once your balance matters to you.
See the checks for yourself
18+ only. Self-custodial: you keep your keys. 0.5% per trade, no minimum. Memecoins can go to zero.