This Mishnah is going to talk about a situation where you see a nursing animal. Could we assume that it has already given birth, which would exempt its next child from the laws of Bechor (firstborn)? Mishnah says, if somebody—Rabbi Shimon Ben Gamliel says, if somebody buys from an idol worshiper an animal that was nursing a young animal, he has taken this animal, but he doesn't know whether it gave birth to its first offspring or not. It's nursing, but there are scenarios where an animal could be nursing even though it never actually gave birth. There are situations like that. The fact that it's nursing isn't necessarily proof that it gave birth.
But the Mishnah says we don't have to be concerned that the newborn animal nursing from it is the offspring of another animal and that this one is just being nice and giving it to drink. Rather, we assume that the animal that's nursing is her own offspring, and therefore the next offspring will not be a Bechor.
A Jew enters among his flock of animals. He sees the animal that just gave birth for the first time. The animals are nursing newborns. Those that didn't just give birth for the first time, but gave birth previously, are also nursing newborns, but he doesn't know whether each mother is nursing her own newborns. He says that a Jew enters among his flock of animals, and therefore the next offspring will not be a Bechor. He doesn't have to be concerned that maybe the offspring of the animal that just gave birth for the first time is nursing from the other animal that gave birth previously, or that the offspring of the animal that gave birth previously came to nurse from this animal that just gave birth for the first time. We could assume that every animal is nursing from its mother, and we don't have a case of questionable Bechorot. We know that the ones who gave birth for the first time are nursing their own children. So their little ones that are nursing from them are the Bechorot. We assume that every animal is nursing from its own mother. The male offspring that are nursing from the first-time mothers are definite Bechorot, and those nursing from the mothers that have given birth previously are not considered to be Bechorot.