The blockchain is a decentralized, distributed ledger that is used to record transactions across many computers so that the record cannot be altered retroactively without the alteration of all subsequent blocks and the consensus of the network.
Each block in the chain contains a record of multiple transactions, and once data has been recorded on the blockchain, it is very difficult to change or remove it. This is because each block is connected to the block before it and the block after it, forming a chain of blocks, or a "blockchain." This means that anyone can view the history of transactions on the blockchain, but no one can change or delete past transactions.
To add a new block to the chain, a group of transactions must be validated by the network. This process, known as "mining," involves solving a complex mathematical problem. The first miner to solve the problem gets to add the next block to the chain, and in return, they receive a reward in the form of cryptocurrency.
The security of the blockchain comes from this decentralized, distributed nature. Because the network is spread across many computers, it is very difficult for any one person or group to manipulate the data. Additionally, the use of complex mathematical problems and cryptography helps to ensure that the data on the blockchain is secure and cannot be altered.