A blockchain is essentially a distributed ledger of transactions. To better understand this, imagine a spreadsheet. On this spreadsheet you list every single Bitcoin transaction – ever.
Alice sends 5 bitcoin to Bob. Bob sends 1 bitcoin to Chris. Chris sends 0.15 bitcoin to Dan. On and on from the very beginning of Bitcoin and without end. It would be a very big spreadsheet.
To make it more manageable, this spreadsheet is separated into smaller pieces or “blocks” and each block is linked to the previous block to ensure everything stays in the correct order. When these blocks are linked together, they form a chain.
See what we did there? A chain of blocks, more commonly known as a “blockchain”.
While this may seem like a simple concept, it has far reaching applications.