A blockchain is like a database that records transactions in a secure and transparent way. Imagine a list of lists of transactions. A single list is called a block. In the next list it links back to the previous list, or block. This linking results in a sequence of blocks, the blockchain.
This chain is decentralized, meaning it's spread across many computers worldwide, ensuring no single entity has control, but instead it is usually written to by a majority consensus. The decentralized nature and cryptographic security make it difficult to alter past transactions, providing trust and security in the system.