Two-factor authentication (2FA) is a way of authenticating you through the use of two different factors. These factors are typically something you know (e.g. a password) and something you have (e.g. a mobile phone). 2FA adds another layer of protection to an email account as it means you need both of these factors to access the account.
To enable 2FA:
1. Click on the Options Gear icon.
2. Click on the Security tab.
3. Click on the Two-Factor Authentication tab.
4. Click on Two-Factor Authentication... to enable it.