A secret word (i.e. a password) is one way of limiting access to something (email, bank, club, etc.). If you don't know the secret, you don't belong - but a secret can be found out. In the physical world, a secret may be discovered by eavesdropping, or by someone sharing the secret. In the online world the password may be stolen by viruses and other malware, or through digital eavesdropping. So while a strong password is a good start, by itself a password could be compromised.
Multifactor authentication is combining a secret with something else - often a smartphone or a keycard. If an attacker steals the password, it does them no good unless they also have the cellphone, or keycard, or whatever the second factor is. Granted a determined adversary may still overcome this hurdle, but it is exponentially harder than simply stealing a password.
Note that a second secret (such as the "password reset questions" that some websites ask you to set up) is not the same thing as multifactor authentication. If one secret can be found out, so can another (especially if the second secret is something easy to lookup, such as your mother's maiden name or the elementary school you attended). True multifactor authentication means something from at least two of the following categories: something you know (such as a password); something you have (such as a cellphone or a key); and something you are (such as a fingerprint or a retina scan).
Now that you know what multifactor authentication is, go back to my original post and see how it is implemented by a handful of popular online services!
