The textbook definition of a Blockchain goes something like this:
A blockchain is a decentralized, distributed ledger that allows data to be stored in a secure, tamper-proof way. In essence, it is a database that contains a continuously growing list of records, called blocks, which are linked and secured using cryptographic algorithms.
Each block contains a cryptographic hash of the previous block, a timestamp and transaction data. Because each block is linked to the one before it, it creates a chain of blocks, hence the name "blockchain."
Once data is added to the blockchain, it cannot be deleted or modified, ensuring its immutability and making it an ideal platform for applications that require a high level of security, transparency and accountability. Blockchains are commonly used in cryptocurrencies, but can also be used in a variety of other applications, such as supply chain management, voting systems and medical record keeping.
With that said...
What this means to you and me is closer to this:
A blockchain is like a digital book that keeps records of transactions, like how much money someone sends to another person. But instead of being owned and controlled by one person or organization, it's spread out across many different computers around the world.
Because it's spread out, it's much harder for someone to hack or cheat the system. It's also designed so that once something is written in the book, it can't be changed or erased, which makes it very secure.
People can use blockchains to do things like trade money, buy things online or keep track of important information like medical records.