For the Bitcoin business, see Coinbase (business).
The coinbase is the content of the 'input' of a generation transaction. While regular transactions use the 'inputs' section to refer to their parent transaction outputs, a generation transaction has no parent, and creates new coins from nothing.
The coinbase can contain any arbitrary data. The genesis block famously contains the dated title of an FT article:
The Times 03/Jan/2009 Chancellor on brink of second bailout for banks