This mishnah is the practical application of the previous mishnah.
Mi'un versus a get:
"HaMema'enes ba'ish" - a minor who performed mi'un against her husband: he is afterward permitted to her relatives and she is permitted to his relatives, and she is not disqualified from marrying into the kehunah, since she is not a divorcee. There was never a real marriage here at all; the marriage is annulled retroactively, and we view it as though it had never existed, as we mentioned in the previous mishnah.
"Nasan lah get" - if he gave her a get, he is forbidden to her relatives and she is forbidden to his relatives, and even though this is only a get on the rabbinic level, it disqualifies her from marrying a kohen.
Returning to the first husband:
As a rule, a woman who left her husband and married another man may not return to the first. But our mishnah describes two cases whose laws differ from one another:
Mi'un after a get: He gave her a get and then took her back while she was still a minor, and afterward she performed mi'un against him. She then married another man and was widowed or divorced from him - she is permitted to return to her first husband. Even though the first time the marriage was ended by means of a get, the mi'un that followed has the last word: the marriage is completely annulled and it is as though it never took place, and consequently even that first get was meaningless, so we do not view her as one who left a marriage on the basis of a get.
A get after mi'un: A minor performed mi'un against her husband, and he then took her back, and afterward he gave her a get, and she married another man - it turns out that her final departure from him was on the basis of a get. If she was widowed or divorced from the second husband, she is forbidden to return to her first husband, since she left him with a get, and we view that first marriage as a full marriage.
This is the rule:
"Get achar mi'un" - when the get comes last, she may not return to him.
"Mi'un achar get" - when the mi'un comes last, she may return to him, since we view the first marriage as though it never existed at all.