It is a method of confirming a user's claimed identity by utilizing a combination of two different factors: 1) something they know (e.g. their password), 2) something they have (e.g. a one-time password, a card, a token), or 3) something they are (e.g. biometrics).
A good example of two-factor authentication is the withdrawing of money from a ATM; only the correct combination of a bank card (something that the user possesses) and a PIN (personal identification number, something that the user knows) allows the transaction to be carried out.
Your password is the only gatekeeper of your zebrix account. Imagine if your password is accidentally known by a malicious person…