Two-factor authentication, or 2FA as it's commonly abbreviated, adds an extra step to your basic log-in procedure. Without 2FA, you enter in your username and password, and then you're done. The password is your single factor of authentication. The second factor makes your account more secure.
Here is a link to a YouTube video that explains the basics of Two-Factor
Redeeem requires 2FA on important features like payouts, and optionally at login.
Set up 2FA on your Security Page.
Download links to Google Authenticator Application: