Two-factor authentication adds extra security when you log in to your account. It involves installing on your mobile device (usually a phone) software which displays a code required to log in. This code changes repeatedly, so even if someone finds out the code you used to log in, the code will expire in a moment.
Thanks to this extra security feature, unauthorised persons will not be able to log in to your account, even if they find out your password. In order to log in, they would also have to get access to your phone. As two different devices (computer to enter the password and a phone to generate codes) are required to log in to your account, your account is much better secured.