A block is a collection of data that is stored on the blockchain. Each block contains a number of transactions, along with other information such as a timestamp and a reference to the previous block.
Once a block is added to the blockchain, it becomes a permanent part of the network and its contents cannot be altered. This is because each block is linked to the previous one through a cryptographic hash, which creates a chain of blocks that is resistant to tampering.
The process of adding a new block to the blockchain is called mining. Miners compete to solve a complex mathematical problem in order to add the next block to the chain and earn a reward for their efforts. This process helps to secure the network and ensures that the contents of each block are accurate and tamper-proof.