A block is a collection of transaction data that forms the basic element of a crypto asset like Bitcoin. When a Bitcoin transaction occurs, information related to the transaction is collected and when the data reaches a predetermined size, it is combined with other Bitcoin transactions and then organized into a block. The block is processed by miners and verified for accuracy.
When a transaction goes into a block and is verified, that's when the Bitcoin transaction gets 1 confirmation. Blocks can also be referred to as data structures in a blockchain database, where transaction data in a crypto asset blockchain is permanently recorded.