Two factor authentication (frequently shortened to just "2FA") means that you'll use a username and password, plus an additional form of identification in order to sign in to a resource. Thus, you're using two factors to authenticate your self into a service. Duo has made a very brief video that explains this concept very well.
Requiring two factors to sign into an IWU resource means that if a "bad guy" or "threat actor" somehow manages to steal a username and password they still won't be able to log into that person's account. In short, having two factor authentication in place keeps your account safe, and helps keep IWU's data safe.