The Red herb is one of three different herbs that can be found growing in pots around the Arklay Mountains and Raccoon City, along with Green and Blue Herbs. The Red Herb is useless on its own (in most titles), but triples the healing effect of the green herb when combined with one.
The herbs were either transplanted to Europe and Africa by Umbrella (unless the plants originally came from one of these locations), as they appear in the fourth and fifth numbered games in the series, fulfilling the same function.
In Resident Evil Outbreak, the herb can do something alone; It can stop your viral meter for about 30 seconds.
In Resident Evil Gaiden, the Red Herb directly heals damage. Its description says "This herbal compound will restore heavy damage". In this game, the herbs look mixed (they are in a powder on some paper), but there is no mixing. The herbs are not called mixed herbs, but rather Green, Yellow, Red, etc. and are their expected colors (not the altered colors of mixed herbs), so there would be no reason to assume they are actually just mixed herbs. Instead, it would seem in this title that the herbs just come in powder form.
In Resident Evil Revelations 2, its effect is slightly different. Instead of triple the effect, it triples the amount of a green herb, making from 1 green herb to 3 green herbs.
They can be found in almost about every title except Resident Evil: The Umbrella Chronicles, Resident Evil: The Darkside Chronicles, and Resident Evil Revelations, whose game mechanics do not require them.