A blockchain is a record of transactions grouped into "blocks", each cryptographically linked to the one before it, forming a chain. Thousands of independent computers (nodes) hold a copy of this ledger and follow shared rules to agree on what's valid, which is what makes the history extremely difficult to tamper with after the fact.
The key idea isn't the word "blockchain" itself, but what it enables: a group of people who don't know or trust each other can still maintain an accurate, shared record of who owns what, without relying on a single company or institution to keep the books honestly.