A blockchain works like a large book on the internet. This book keeps track of how much money everybody has. A single block in the blockchain is like a single page in a book. You can fit only so many words on one page before the page is full and you have to use the next one.
The blockchain documents every transfer of money. This ensures that no one can spend the same set of his or her money twice. If you have one ZEN and send it to a friend or an online shop, the recipient is now in control of that ZEN and you cannot use it anymore.
Blockchains solved a major problem in the digital world. When I send you a picture in an email, I actually just send you a copy of that picture and afterward, we both have a copy. I could send the same picture to another person later on. Blockchains allow digital things to exist in only one location at a time. This is one of the main reasons blockchains are innovative. Digital money would not work if I could send you a copy of my money and spend it a second time later on.