What is "She'er"?
Rashi, Ramban #1 (Pasuk 9, citing Targum Onkelos), Rashbam, and Targum Yonasan: Sustenance. 1
Ramban #2 (Pasuk 9): Proximity of flesh at the time of intimacy. 2
What is "K'sus"?
Rashi, Targum Onkelos and Targum Yonasan: Clothes.
Ramban (on Pasuk 9): Bedding. 1
Rashbam: It means ornaments.
What is "Onah"?
Rashi, Ramban (on Pasuk 9), Rashbam, Targum Onkelos and Targum Yonasan: Intimacy, in its appointed time. 1
Rashbam: A place to live. 2
Ramban: The Torah is warning the son here, that in the event that he marries another wife, he may not relegate the Meyu'edes to the status of a concubine, by being intimate with her only casually, clothed and on the floor, whilst he is intimate with his second wife regularly, on a bed and without clothes.
Like the word "Me'onah" (the 'Mem' of which is not intrinsic [See Rashbam]).
What do we learn from "Im Acheres Yikach Lo
Seforno: One may not marry more women if he cannot provide his obligations to them (Yevamos, 65a).
Kidushin 16a: This equates an Amah to an Arusah. Just like a document can be Mekadesh, it can acquire an Amah.
Rashi: If one marries a woman in addition to the Meyu'edes, he must still fulfill his obligations to the Meyu'edes.
Aruch ha'Shulchan (EH 26:8, based on the Rambam): If he is Mekadesh a woman, these obligations come after Nisu'in. However, a Meyu'edes is like a concubine; these obligations do not apply. This is like one opinion in the Mechilta.