For each slot in an Epoch:
The Cardano network selects a Stake Pool to be a Slot Leader, based on the Proof of Stake consensus protocol (Ouroboros).
The Slot Leader then collects valid Cardano transactions from the network, fits them into a block, and adds that block to the Cardano blockchain.
This will earn rewards for the Stake Pool and all those ADA holders who have delegated their stake to that Pool.
More from Cardano.org
More from IOHK Research