A "coinbase transaction" is the unique transaction that occurs when a miner successfully adds a block to the blockchain. Its purpose is to reward the miner with some amount of the cryptocurrency in order to incentivize people to mine in the first place.

For example, with Bitcoin, approximately every ten minutes miners compete for the right to add a block to the blockchain. When one wins, a coinbase transaction occurs to create new Bitcoin (currently 6.25 BTC) to give them to the miner as a reward.