A coinbase transaction is a special Bitcoin transaction placed first in each block. It allows the block producer to claim the block subsidy and fees from the block's other transactions. The protocol term is separate from the company named Coinbase.
Where its value comes from
Unlike ordinary transactions, a coinbase transaction does not spend an existing transaction output. It uses a special input structure, and its outputs must not claim more than the allowed subsidy plus collected fees. Full nodes reject a block that overclaims, even if its proof of work meets the target.
Maturity before spending
Coinbase outputs must satisfy Bitcoin's 100-block maturity rule before they can be spent. This reduces the risk of reward outputs circulating before a competing chain makes their originating block stale. Maturity is measured in blocks, not a guaranteed number of hours.
More than issuance
Coinbase data can carry the block height and miner-selected data; changing permitted data also gives miners new header-hash candidates through the Merkle root. Outputs may divide the reward among several recipients. A valid block can contain only its coinbase transaction, and a later mining-pool payout is not automatically itself a coinbase transaction.