Rashi and Ramban #1: It means that she remains ¹ with the blood of her Taharah. ²

Ramban #2: It means that for thirty-three days, she may be intimate with her husband - even if she sees blood. ³

Ramban #3: It means that during the thirty-three Y'mei Taharah, although she is permitted to her husband, she should 'sit' (on the seat of Nidus ⁴ on which she sat when she gave birth) and remains forbidden to eat Kodesh or to enter the Beis-ha'Mikdash. ⁵

Ramban #4 (citing the Sifra in Tazri'a): It comes to preclude from Tum'as Zivus a woman who sees blood during Koshi (the pangs of childbirth) during the eleven days of Zivus. ⁶

Rashi and Ramban: Refer to 12:4:1;2.

Seforno: Because any blood that she sees during the thirty-three days of Y'mei Taharah are not from a current Veses (period), but from earlier periods from the time of her pregnancy, in which case the blood has already spoiled and lost its Tum'ah.

Rashbam (according to Levi, Nidah 35b): Because it is not the blood of Nidus, but blood that flows from an independent source.

Nidah 35b: Even though the blood is from the same source as Dam Nidah, the Torah decreed that it is Tahor.

Rashi: I teaches us a prohibition against eating (not touching) ¹ Kodesh during the days of Tohar; the word "be'Chol" comes to include Terumah. ²

Moshav Zekenim (citing Yevamos 75a): It is also a prohibition against touching Terumah. ³

Zevachim, 32b: It teaches us that, just as touching part of an object renders the entire object Tamei, ¹ so too, is one Chayav even for placing just one's hand inside the Mikdash. ²

Ramban #1 (citing the Ibn Ezra, according to R. Yishmael in Nidah 30a): After the days of Tum'as Leidah, the Y'mei Tohar extend until forty or eighty days, which correspond to the days in which a baby boy and girl ¹ (respectively) are formed.

Ramban #2 (according to the Chachamim in Nidah, 30a): "Y'mei Taharah" actually means pure in the sense of clean, ² and the Torah decrees seven days of Tum'ah, because the woman generally sees blood throughout the seven days following the birth and thirty-three additional days, the time it takes for her body to dry; ³ and double for a girl due to the excessive moist and coldness that a girl creates in the womb, which inevitably takes longer to dry. ⁴

Moshav Zekenim #1 (in Pasuk 5): A fetus is formed facing down. Before birth, a girl flips over to face up. This affects the womb causing twice as much pain as birth of a boy.

Moshav Zekenim #2 (in Pasuk 7): During labor pains, a woman swears that she will never again be intimate with her husband (Nidah 31b). When a boy is born, all are happy, and after seven days she regrets her Shevu'ah. When a girl is born, she regrets only after fourteen days. ⁵

Ramban: Refer to 12:4:5:1 & 2.

Moshav Zekenim #1 (in Pasuk 5): A fetus is formed facing downwards. Before birth, a girl flips over to face up. This affects the womb causing twice as much pain as the birth of a boy.

Moshav Zekenim #2 (in Pasuk 7): During labor pains, a woman swears that she will never again be intimate with her husband (Nidah 31b). When a boy is born, and everyone is happy, she regrets her Shevu'ah after seven days. Whereas when a girl is born, she regrets it only after fourteen days. ¹

Ramban: The basic distinction is between Kodesh (which is forbidden) and Chulin (which is permitted) - and her husband falls under the category of Chulin.

Riva and Hadar Zekenim: The implied Heter pertains exclusively to Terumah ¹ (which the Torah also calls "Kodesh"), but not to entering the Mikdash or eating Kodshim.