This is completely new to me. Super interesting!
Koji is cooked rice and/or soya beans that have been inoculated with a fermentation culture, Aspergillus oryzae. [via]
In addition to marinating chicken, koji adds umami to to vegetables and fish and can be used in sake, soy sauce, miso, sauces, salad dressings and sake. SFGate has more info and a few recipes to try out.