To active the new functionality (and you should, as soon as possible), users are advised to visit their account settings page, scroll down to Account security, click on the link (and follows instructions) to "add a phone," then select "Require a verification code when I sign in."
Once login verification is enabled, you'll receive a six-digit code via text message whenever you attempt to sign in -- that code will have to be entered along with your password in order to access your account.
"This release is built on top of Twitter via SMS, so we need to be able to send a text to your phone before you can enroll in login verification (which may not work with some cell phone providers)," Twitter's Jim O'Leary wrote in a blog post. "However, much of the server-side engineering work required to ship this feature has cleared the way for us to deliver more account security enhancements in the future. Stay tuned."