Blockchain is a chain of interconnected blocks of information that is written in an encrypted form and maintained as a transaction ledger on a network of connected computers. Any information written on this ledger has to be agreed through a consensus mechanism. To be valid, each block contains reference to the previous block to form a chain of information stored in blocks.
This can also be done manually without using computers. When stripped to its core, blockchain is really just a sophisticated ledger system. People can take responsibilities to duly update and maintain copies of a ledger in their custody, based on an agreed set of rules to reach consensus about what and how to record the information.
In my opinion, the story on this link explains it very well.
https://hackernoon.com/how-to-run-a-blockchain-on-a-deserted-island-with-pen-and-paper-899949ec555b