Two-factor authentication (2FA) adds a second layer of security to your account. Even if someone learns your password, they cannot sign in without your authenticator app.
To enable it, go to Account → Two-factor authentication and choose Enable. Scan or type the secret into an authenticator app such as Google Authenticator, Authy or 1Password, then enter the six-digit code to confirm.
You will be shown ten one-time backup codes. Store them somewhere safe — each can be used once if you lose access to your authenticator app.
If you ever need to turn 2FA off, you can do so from the same screen after confirming your identity.