A block is the basic unit of a blockchain. It contains a list of transactions, called a ledger, that have been verified and added to the chain. Blocks are connected to other blocks by using hashes, which are a type of digital fingerprint. This makes it impossible to change a block without changing all the blocks that follow it in the chain. Because of that, it's impossible for a malicious hacker to edit it (for example to create or delete past transactions). Every block is immutable and the chain keeps going. That's why it's called Blockchain.