What is blockchain and why Steemians rely on this technology

For me blockchain is simple. I don't understand much of it.. which makes it simple.

My girlfriend asked me whats the link between blockchain and Steemit.
here is the simple explanation: Every Steemian has an office in one of these big towers.
When you create your unique username and password, you create a key to an office.
Only you have this key.. thats why passwords can't be recovered.

In that office you run your bank.
This technology is just like banking and it takes place on the internet.. and a skyscrapper looks like banking in the reality.. at least according to me :)