https://www.gamespot.com/articles/how-red-dead-redemption-2s-stats-health-and-cores-/1100-6462201/

In terms of scale, scope, gameplay systems, and interactivity, Red Dead Redemption 2 feels like it's shaping up to be more of an in-depth role-playing game than the open-world action game we expected. And just like with most RPGs, there's a whole bunch of stats to consider when thinking about your character. In addition to a traditional health meter, Red Dead Redemption 2 protagonist Arthur Morgan's well being is governed by Cores, which take some explaining to get your head around.

Although health, stamina, and Deadeye meters are separate from Cores, how they perform depends on the state of a corresponding Core. For example, the health Core dictates how quickly health regenerates, and likewise for stamina. If you're low on health but the corresponding health Core is has been maintained well, the rate at which you generate health will be increased.