Short & Sweet,
a blockchain is a DECENTRALIZED internet.
Medium & Technical,
a blockchain is a DECENTRALIZED internet protocol hosted by a network of computers. These computers are rewarded for their hosting expenses with a portion of the controlling power in the computer network known as cryptocurrency.
Long & Simple,
To understand blockchain you just need to understand the capital lettered difference between "Internet and internet" and "Bitcoin and bitcoin".
Capital lettered "Internet" means the computer network using the Internet protocol suite (TCP/IP). Moreover, TCP/IP just means a type of internet protocol. It is the type of internet protocol the vast majority of popular websites use.
Lowercase lettered "internet" is defined as an interconnected computer network. Therefore, internet with a lowercase "i" is simply an interconnected network of computers.
Blockchain is also an interconnected network of computers, but blockchain follows a decentralized internet protocol. The protocol ensures all data across the blockchain is perfectly in sync.
Computers on the blockchain are rewarded for their hosting expenses with a portion of the controlling power in the computer network known as cryptocurrency.
Therefore, users of the blockchain in total own the blockchain through their cryptocurrency share. They then use their cryptocurrency to post, like, message and more on all the blockchain's websites and applications.
So when someone asks you "what is blockchain?", say "a decentralized internet".