BHA means Beta-hydroxy acid. These acids can be dissolved in oils and are used in cosmetics for exfoliation.
BHAs can help to clean the skin, free its pores, and remove dead cells from above it. BHAs but also AHAs are a good choice if your skin is dull and has an uneven tone hyperpigmentated.
Though there are many acids that can be defined as BHAs by a chemist, it actually almost always salicylic acid, which is used in cosmetics. Citric, malic and tartaric acids belong to BHAs and AHAs, but act more as AHAs.
BHAs might cause irritation. We recommend starting with lower concentration products (under 2%) and using it every other day to avoid a skin reaction.