The Green Herb is a medicinal, leafy plant which is generally grown indoors, in pots. Initially it was found in the Arklay region in the United States, but it can now be found in various places across the globe. They are quite commonly found and can be kept in storage for long periods of time.
In general, it can be ingested directly, restoring some strength. It can also be ground up and combined with more Green Herbs for a more potent effect, or with different Herbs to produce different effects entirely.
The Herbs can also be converted into easy to ingest Tablets, providing the same effects.
The Green Herb is a staple of the franchise, and as such appears in every game. It's generally designed to heal the player by between 20% and 30% by itself, between 50% and 60% if combined with another, and 100% if three are combined. There are other combinations available by using a Green Herb with different colour Herbs.
Specifically in
Resident Evil 6, Herbs collected are converted into tablets. 1 Herb = 1 Tablet, 2 Herbs = 3 Tablets, and 3 Herbs = 6 Tablets. One Tablet recovers one section of health, a maximum of 6.