What is Salicylic acid?
Salicylic acid known as beta hydroxy acid (BHA) it is a multi-tasking ingredient; it is a must for anyone prone to acne, due to its exfoliating nature. As an anti-irritant, it has the ability to soothe stressed skin, reduce redness and work as an anti-inflammatory to calm breakouts and angry skin.  It is derived from the bark of willow trees.
What does it do?
Salicylic acid breaks down the bond that keeps dead cells together which otherwise clog pores leading to blackheads and whiteheads as well as reducing sebum secretion. It’s also been proven to have renewing properties, helping to firm and plump the skin by promoting new cell turnover. Salicylic acid has the ability to provide a deep clean because it can penetrate deep into the pore lining. This means it’s able to exfoliate inside the pore as well as the surface of the skin. 
On application you will experience some stinging and perhaps peeling, dry, and reddened skin. But, it’s actually safe to use on sensitive skin and even has anti-inflammatory properties. Over time, your skin will become accustomed to it and the side effects will lessen.
1.Beta Hydroxy Acids in Cosmetics”. Archived from the original on 2007-12-21.
 Madan RK; Levitt J (April 2014). “A review of toxicity from topical salicylic acid preparations”. J Am Acad Dermatol. 70 (4): 788–92
 Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 64.