2-factor authentication adds an extra layer of security to your account. After you enroll, instead of logging in with just your password, you'll be also asked for a code from the 2nd factor -- typically your phone -- before you'll be able to log in. This way, if someone steals or guesses your password they won't be able to log in to your account, as they'll still need that 2nd factor to log in.
All accounts made directly with Action Network using a username and password are eligible for 2-factor authentication. (Accounts made with social media logins are not, though both Facebook and Twitter offer 2-factor authentication of their own, which you can use to secure those accounts.)
You can enable 2-factor authentication by editing your profile in the menu in the upper right when logged in.