A blockchain is a decentralised network of peer to peer nodes that replicate a digital transaction ledger and is made up of blocks of securely encrypted data which are chained together and dependent on each other. This means that there are several exact copies of the same data stored in several different locations around the world that people can publicly access and verify.

The below diagram shows how a blockchain works within a specific scenario of a Bitcoin transaction.

Sources:

https://blockgeeks.com/guides/what-is-blockchain-technology/

https://www.coindesk.com/information/what-is-blockchain-technology/

https://itnext.io/what-is-blockchain-technology-735a19951ff9

https://youtu.be/r43LhSUUGTQ

https://youtu.be/l9jOJk30eQs

https://youtu.be/k53LUZxUF50