الصَّابِرُ comes from the root ṣ-b-r and is an active participle, “one who perseveres.” الْقَابِضِ, from q-b-ḍ, is likewise an active participle, “one who grasps.”
The prefix كَـ introduces comparison. عَلَى الْجَمْرِ literally means “upon the ember.” A physical image is used to communicate moral difficulty.
The virtue is not difficulty for its own sake; it is remaining rightly committed عَلَى دِينِهِ—to one’s religion.