Two factor authentication, also known as multi-factor authentication (2FA or MFA for short) is a principle of combining something you know (i.e. your password), with something you have (i.e. your phone). Various examples of 2FA include receiving a push notification on your phone for approval, receiving a text message with a code, and more. 

View this short video from Duo Security for a quick overview: https://www.youtube.com/watch?v=0mvCeNsTa1g