A block is an individual unit of a blockchain. Each block contains the hash of the previous block, confirmed transactions, and a number called a nonce. Someone creating a block must find a nonce such that the hash of the block is below a certain threshold (the target), which can only be done by trying out nonces one after the other until one that produces a desirable hash is found, and is harder the lower the target is. The reason why block creation is made deliberately difficult is to prevent someone from spending bitcoins and then creating and pushing his own blockchain that doesn’t contain the transaction that shows that the bitcoins are spent, effectively erasing that record and allowing him to spend them twice. When a valid block is created, it is distributed through the network and work on the next block starts.