However strong your password is, it can still leak in a data breach or get typed into the wrong site. Two-factor authentication adds a second lock to your account: when you sign in, you enter your password plus a 6-digit code from the authenticator app on your phone. The code changes every 30 seconds, so even someone who knows your password can't get in without your phone. In Settings it's called Two-Factor Authentication (2FA), and you can turn it on with any plan.
If you don't have an authenticator app on your phone yet, install one first; you'll use it to scan a QR code in the next step. When you're ready, click Start Setup.
As soon as the account is added, the app starts showing a 6-digit code that refreshes every 30 seconds. Type the current code into the 6-Digit Code field and click Verify and Enable.
The section now says 2FA Active, and you can see how many recovery codes you have left. If you're running low or have lost them, click New recovery codes to get a fresh set. All you need is the current code from your authenticator app; your old codes stop working from that moment on.
You don't have to enter a code every time: the browser where you entered it is remembered for 30 days, and you won't be asked for a code there during that time. If your phone isn't with you, click Phone not with you? Use a recovery code and enter one of the codes you saved. That's it; your account is now one step safer.