Twitter is introducing a new security feature to better protect your Twitter account. When you sign in to twitter.com, theres a second check to make sure it’s really you. Youll be asked to register a verified phone number and a confirmed email address. With login verification enabled, your existing applications will continue to work without disruption. If you need to sign in on other devices or apps, visit your applications page to generate a temporary password to log in and authorize that application.”]
Source: https://blog.twitter.com/official/en_us/a/2013/getting-started-with-login-verification.html