A block is a collection of transactions that occur on the Bitcoin network. These blocks are linked together chronologically to form a blockchain. Most Bitcoin blocks include around 2700 transactions and can be up to 4MB in size.
A block can only be added to the blockchain if it has a hash that satisfies Bitcoin’s Proof-of-Work requirement and includes the previous block’s hash. The inclusion of the previous block’s hash in a block ensures that no block can be altered without invalidating subsequent blocks. This trait is due to the nature of hash functions, which are deterministic and random. This system gives Bitcoin immutability.
For example, if a transaction in Block #400 is altered, the hash of Block #400 will change, invalidating the Proof-of-Work for Block #400, but also making Block #401 invalid, because Block #401’s “previous hash” parameter no longer matches the hash of Block #400. This change will ripple forward, disconnecting all blocks after Block #400. This feature ensures that once a block is added to the blockchain neither it nor any transactions included within it can be altered.