A blockchain is a peer-to-peer Database (aka Ledger) of transactions arranged in a chronological order through encryption. Any data recorded in the Blockchain is immutable in a way. Immutability adds the trust on the recorded data which makes the Blockchain powerful for certain use cases.
Miners add a Block into the Blockchain by adding the new transactions into it and sign it cryptographically after solving a computation challenge. This process is called Mining. Miners get rewarded by for this work through transaction fees in that Block which is paid by all the users while proceeding with the transaction.