Multi-factor authentication (MFA), sometimes referred to as two-factor authentication (2FA), is a method where users obtain access by providing two separate factors to identify themselves. Verifying your identity using a second factor (like your phone or other mobile device) prevents anyone but you from logging in, even if they know your password.
For more information and frequently asked questions, read IT Security's page on Duo MFA
Authenticating with the Duo Prompt - the Duo prompt lets you choose how to verify your identity each time you log in.