When you enable this feature, it adds an additional layer of security to your account. When logging in to GitHub, after providing your username and password, you will be asked for a two-factor authentication code that is delivered to your mobile device via SMS or a free two-factor application. This additional step ensures that a malicious person who has discovered your password will not be able to log in to GitHub as you.
Seeing a lot more two-factor authentication being offered on the net. This can really help make your accounts safer, but you’ll have a problem if you don’t have your phone. GitHub provides a number of recovery options including backup SMS numbers and recovery codes. If you’re going to use 2 factor auth make sure you have recovery options setup.