Two-factor authentication (2FA) is a security process which requires users to provide two means of identification before accessing their account.
The two levels of security are your password and a unique special code generated by an authentication app, installed on your smartphone. The two most popular authentication apps are Google Authenticator and Authy.
If you have 2FA enabled, you will receive a temporary 6-digit code which can be used only for a very short period of time: about 20 seconds. After 20 seconds, the code will expire and a new code will be generated. The requirement to enter both your password and this 6-digit code makes your account better protected. Even if your password is compromised (make sure to never reveal it to anyone!), your account cannot be accessed without direct access to your mobile device.
To read about how 2FA works, feel free to check out this Wikipedia article about 2-factor authentication.
Note: for larger transactions, you may also be asked to confirm via 2FA.