First treatment essences are products that typically contain some form of yeast (or other) ferment, and they are meant to be applied prior to any toners or other essences. They are usually known for their ability to reduce redness and improve skin health and function. As a rosacea sufferer, anything that can possibly reduce redness is going to be an attractive option.
My first product in this category was the Nature Republic Saccharomyces The First Treatment Essence. I really enjoyed that product, but it was unfortunately discontinued and I was put on a mission to find a replacement. The COSRX Whitening Power Essence has been my first attempt.