Login verification is a feature that helps you keep your account more secure. Instead of relying on just a password, login verification introduces a second check to make sure that you and only you can access your Twitter account. Only those who have access to both your password and your phone will be able to log in to your account.
Now, when you log in to your account on twitter.com, a six digit code will be sent via SMS to your phone. Enter the code when prompted and you're in!
After you enable login verification on your account, you’ll need to use a temporary password to log in to Twitter on other devices and applications that require you to enter your Twitter password.
To use a temporary password:
I didn't receive the code.
It says my phone is associated with a different account.
I lost my phone!
For more information about keeping your account secure, check out this help article.