Here's Molly. She goes to Facebook, where she enabled 2FA, and proceeds to log in. 

After entering her credentials, Facebook prompts Molly to check Verigator for a secret code, the second authentication factor.

Molly enters the code she sees on the Verigator app into Facebook's 2FA login screen. 

Success! Molly has logged into Facebook and has proved that it's actually her logging in (not a hacker or a friend that may know your password).