ELI5 Crypto Series: #1 What is the Blockchain?

hey fellow Steemittors,

I am starting an 'explain like I'm five' (ELI5) series focused on cryptocurrency (crypto). The ELI5 format originated on Reddit and in my opinion, is an extremely effective and efficient format for grasping the understanding of a subject at it's most basic level, in a time and resource efficient manner.

Please feel free to suggest interesting and relevant ELI5 questions in the comments and I will try and include them into this series. I'd also appreciate your general feedback on implementing ELI5s, or other formats here on Steemit. My hope is that we can make this series into a useful tool for people who arrive via google and are looking for basic understanding of the crypto world our society is slowly shifting into.

So finally after all this introduction, here's my question:

I would like to explain the blockchain to my parents.
The only ELI5 on reddit I could find is 2 years old and not age-appropriate at all!

ELI5: What is the blockchain?