Also called 2-Step Verification or 2FA.
In order to enhance the security, various websites and applications have started to use a second layer of verification, on top of the standard username/password combination.
With crypto exchanges, it has become a standard by now and everyone should be using this.
There are several methods of 2FA, it can be be an extra code (PIN), email or SMS verification (code send to either of them) ...
But the most popular are apps like Google Authentication or Authy. These generate a new unique code every 30 seconds and are considered highly secure. In order for them to work, you have to sync them with each exchange first.