Rashi (27, citing Pesachim 119a): Yishai said this when Shmuel anointed David. Rashi (119a) - David was a shepherd (no one thought that he will be king; Rashbam - and he was Yishai's youngest son). Maharsha - Do'eg and his entourage of Chachamim, who are called builders, despised David; they said that he is not proper to marry into Yisrael. Mar'is ha'Ayin (119a) - once, Yishai told his Shifchah to come to have Bi'ah with him ¹ . She was modest; she told Yishai's wife, who went covertly in place of her. She became pregnant; Yishai had separated from her, so he and his sons thought that the child (David) was a Mamzer from Zenus, until Shmuel revealed the matter. "Bonim" is like Banim.

Rashi: Yisrael were lowly among the nations.

Radak #1: If this refers to Yisrael, they are called "Even" - "Ro'eh Even Yisrael" (Bereishis 49:24). It is a metaphor; Yisrael are primary in the world, and they sustain the world via their Chukim and Mishpatim. They were despised in Galus; the Bonim (kings) despised them and thought that there is no need for them, and thought to eradicate them. In the future they will be the cornerstone.

Tosfos ha'Shalem (Bereishis 28:22:5): There was a rock, that when they wanted to set it in the [wall of] the Mikdash, it made itself fall, because the Mikdash would later be destroyed. Therefore, the builders despised it, until Shlomo swore to it that even if the Mikdash is destroyed, it will not more from is place. They set it in the Western Wall.

Malbim: Kingship is depicted as a building, and the king is the foundation - "Hineni Yisad b'Tziyon Aven" (Yeshayah 28:16). The Bonim despised him then - Shevna and his entourage contested his kingship.

Radak: Just like one uses great, nice stones for the cornerstones, to establish and support the building, so Yisrael will be in the future; they will sustain the world.