Two-factor authentication (2FA) adds an additional layer of security to your account by requiring more than just a password to log in. With 2FA enabled, you will be prompted for a six-digit code when logging in that is generated using a mobile authenticator app, similar to the TAN process used for online banking.
Go to Settings → Two-factor authentication → Activate to configure 2FA. You are then guided through the setup procedure.
In case you do not have access to your authentication app (for example, if the battery is low), you can use one of the recovery codes created during setup. Each code can only be used once. Keep these codes safe and protect them as well as passwords!