A 'block' is a record of new transactions that have been added to the blockchain. These blocks are added to the blockchain in a linear, chronological order, and once a block is added to the blockchain, it is very difficult to alter the data contained within it.

Each block contains a record of multiple transactions, and once a block is added to the blockchain, it is connected to the previous block, forming a chain of blocks. This chain of blocks is what gives the blockchain its name, and it is this chain of blocks that makes it so difficult to alter the data contained within the blocks, since any attempt to alter a block would require altering all of the blocks that come after it in the chain.