"Greyhawk Initative" a variant system of tracking combat for 5E that he introduced at GaryCon 2017 (I was there but missed this bit). Initial reactions online have been muddled and confused. I have no desire to do a rules analysis or gripe about how this bad idea is attached to Greyhawk. Here is a couple places to help you out if you are interested.
ENWorld (including Mearls video)
Tribality (good breakdown of rules)