Due to health reasons, he is resigning from his position as manager.

resigning 是动词 resign 的现在分词形式。resign 由前缀 re-（表示"再次"或"离开"）和 sign（签署）组成，字面意思是"再次签署"或"签署离开"，引申为"辞职、放弃职位"的意思。resigning 在这里表示"正在辞去"的动作。