2-step verification adds more security to your Google
accounts by requiring you to enter a verification code in addition to their
username and password, when signing in to their account. In addition to your username and
password, you'll enter a code that Google will send you via text or voice
message upon signing in.
Even if a password is cracked, guessed, or
otherwise stolen, an attacker can't sign in without access to the user's
verification codes, which only the user can obtain via their own mobile phone.