Multi-factor authentication is where more than one item is used to identify people to computers and networks.
Historically we have used a single authentication method to identify ourselves to our computers and them to recognize who we are. The primary way that was used was passwords. However, we can make our computers and networks much more secure using a multi-factor authentication, usually now it is a two-factor.
When a multi-factor authentication is used usually the requirements are that the two items used come from two of three basic groups. The groups are something you have, something you know and something of you.
Something you have would be a card (your debit card to use the ATM machine), a flash drive, an ID card, etc. Each of them being something you have when you go to log on. However, notice these can be stolen and then someone else has and they can appear to be you. Sending a code to your phone for you to enter fits here also.
Something you know would be things like passwords, PINs, your momís maiden name, the street you grew up on, your favorite color, etc. This information obviously belongs to you but again be stolen by watching you enter codes, you tell someone the information, your significant other (or former) knowing you well, etc.
Something you are could when the computer looks at your finger print and matches, doing a retina (eye) scan to see if same, facial recognition, etc.
You will find more and more systems now not just having you enter a password but requiring one or more or the above. For instance, some systems now require a password and then it sends a text to your phone. You know the password and have to have the phone. If someone knew your password, they could not get in as you (and you would know someone tried) as the code would go to your phone and they would not have, and you have been texted someone is trying to access as you.
Two factor authentication is real now and should make our lives on the computer safer and if done right should not be a lot of extra bother to us and over time will seem normal.