A block on the Cardano network is a group or bundle of transactions that a stake pool creates, verifies, and records on the shared ledger.
At the beginning of each epoch, the Cardano protocol randomly picks slot leaders—pools assigned to create blocks. The protocol uses staked ADA as a form of “lottery numbers” to do this.
When its slot comes, a selected slot leader collects initiated ADA payment and smart contract executions and compiles them into a block before adding them to the ledger.
The process repeats itself after each epoch. Meanwhile, the slot leaders are incentivized through a reward in newly released ADA and the transaction fees.
You may learn more about Cardano stake pool operation HERE.