Q: How can a new computer know wifi passwords?

Recently i installed a new Macbook Air for my wife. Everything installed/setup from ground up. 

Now here's the thing; next time i started it automatically log in to my secure wifi network at home. How is this possible? How can this new computer which has never been in my home, know the password to my Wifi? Even worse, having a look in the Wifi history i can see hundreds of Wifi networks that i have been logged in to in the past - on my other devices! This computer has never been anywhere.

So, it seems something is connecting devices here and sharing this information. What i need to know is; how is this possible?