Salicylic Acid is a Beta hydroxy Acid derived from Willow bark. It is oil soluble and can penetrate into the pores of the skin.
-Fights acne causing bacteria and excess sebum
-Disolves the glue that binds blackheads to skin.
-Draws out impurities rather than drying them out
Apply it on a clean face before applying other products. It's best not to mix many products esp when using a product for the first time so that you can observe how it's working. Also, some ingredients you may use may not mix well with acids. It can be used twice a day as both a day and night serum. It is highly moisturizing and doesn't require much or any layering, hence the recommendation to use it by itself.