My skin is very sensitive to sunlight, but recently it's been tolerating short exposures better.

tolerating 是动词 tolerate 的现在分词形式。tolerate 的意思是"忍受、容忍、耐受"，指对某种不利条件或刺激的承受能力。在这里表示皮肤对阳光的耐受性有所改善。