What were the "Mar'os ha'Tzov'os"?
Rashi, Ramban (citing Targum Onkelos) Seforno and Targum Yonasan: They were the copper mirrors that the women, who in a display of disdain for worldly desires, donated, which were now accepted and used to manufacture the copper basin and its stand. 1
Which explains why later, in Pesukim 39-40, the Torah does not include the basin and the stand among the items that were manufactured from the donations of copper (Rashi and Seforno).
What does "Tzov'os" refer to?
Rashi: It is a reference to the episode in Egypt when they encouraged their husbands, who were worn out from the back-breaking work, to be intimate with them, by bringing them food and drink and enticing them via the mirrors. 1 And they subsequently gave birth to babies in the fields - who ultimately formed 'the hosts of Yisrael'. 2
And what does "asher Tzav'u Pesach Ohel Mo'ed" referring to?
Rashi and Ramban #1: The women now congregated at the entrance of the Ohel Mo'ed in order to donate their mirrors.
Ramban #2 (citing Targum Onkelos), Seforno and Targum Yonasan: They congregated at the entrance of the Ohel Mo'ed to hear the word of Hashem (Seforno) or to Daven (Targum Onkelos and Targum Yonasan),
Why did Moshe initially refuse to accept the mirrors?
Rashi: Because he considered mirrors, tools of the Yeitzer-ha'Ra, to be disgusting.
Then why did Hashem accept them?
Rashi: They were precious in the eyes of Hashem, because the righteous women used them to raise the hosts of Yisrael. 1
Ramban #2, citing Targum Onkelos), Seforno and Targum Yonasan: Because giving away their mirrors was an act of righteousness, in that they overcame the Yeitzer ha'Ra of desire.