The seed (or secret) phrase is 12 or 24 regular words (such as "road coffee watch lipstick" etc) that is both your login and password.
In the backend, these words correspond to a special code (so words aren't random — there's a small dictionary set) which allows anyone take control over your wallet.
This seed phrase can also be encoded as a QR code, which is shown inside your Metamask account.
⚠️ Do not save it to your computer.
⚠️ Do not save it where others can easily see it (ie a sticky note).
⚠️ Do not share your screen with people you don't know — attackers might trick you into showing the QR code containing the phrase.