If you are on Twitter then you are probably aware of the recent compromising of some big time Twitter accounts, the Associated Press in particular and other instances of phishing scams. Users now have the option of having two-factor authentication through SMS to help protect their accounts from hacking and phishing scams. If multiple people use one account (schools, school districts, athletics, classrooms, etc) it will only send the authentication to one phone number.
Two-Factor authentication works by allowing users to require the verification and each time you sign in a code will be sent to your phone which will be needed to access your account. This is not EVERY time you sign in, but when it is a first time from a new device or computer. To learn more, see the short video below.