What does 'u've'Ha'atif ha'Tzon" mean?
Rashi #1 (citing YTargum Onkelos) and Targum Yonasan: It means 'When the sheep were late'.
Rashi #2 (citing Menachem): It means 'When the sheep were covered with wool - and did not therefore need to be kept warm by the males.
What are Mekusharos and Atufim?
Moshav Zekenim (37): Mekusharos are males tied [so they cannot mate]. Atufim are sheep totally wrapped [to protect their wool].
Tosfos ha'Shalem (40:4): Atufim are fat animals that will give birth by themselves. Mekusharos are barren animals.
Hadar Zekenim (38): Mekusharos are born in summer, and Atufim in winter (when the sheep is covered with wool. In summer, it is shorn.)
Ramban (37): Mekusharos are males that constantly follow the females, and are inextricably bound to them - from the Lashon "v'Nafsho Keshurah be'Nafsho" (Bereishis 44:30).The resulting babies are called "Mekusharos" after their fathers. Some say that Mekusharos are strong; their limbs are tightly-knit. Atufim are weaker, and less lustful.
Just like people tie a string around Bechoros (Da'as Zekenim 41) or Bikurim (Rosh 41), Yakov tied around the forelegs of a Bechor.
Rashi, citing Menachem: Ha'Atufim are totally covered in fleece, and are not interested in being heated through mating.