“With harm”: illness, poverty, tribulation, or hardship.

“Can they remove His harm?”: that is, they are utterly unable to do so.

“With mercy”: meaning health, well-being, goodness, and the lifting of affliction.

“Allah is sufficient for me”: Allah alone is enough for me and for anyone who relies on Him.

Overall meaning of the verse: Allah commands His Prophet Muhammad ﷺ to put a question of censure to the polytheists concerning the idols they worship alongside Allah: Do those idols have any power to bring benefit or avert harm? They will inevitably admit their incapacity. Once that is acknowledged, worshiping them besides Allah is rendered invalid.

Relevance of the verse to this chapter: it provides proof for the falsity of shirk. Wearing a ring or a thread for that purpose belongs to the same category, for it can neither remove harm nor prevent it.

Lessons derived from the verse:
1- The invalidity of shirk, for everything worshiped besides Allah owns no power to harm or benefit its worshiper.
2- A warning against wearing rings, threads, and the like to obtain benefit or repel harm, because this is a form of shirk akin to what is sought from idols.
3- The legitimacy of debating polytheists in order to refute shirk.
4- The obligation to rely on Allah alone and to entrust all matters entirely to Him.