Mishnah 2 in chapter 4 of Maseches Nazir follows directly from the end of the previous mishnah, and deals with the relationship between a husband and wife who take vows of nezirus - and with the question of when the husband has the power to annul her vow. The mishnah presents two cases, which differ from one another in the question of which of the two initiated the vow.
The first case - the husband vows and the wife answers amen:
"Hareini nazir, ve'at" - the husband accepts nezirus upon himself, and turns to his wife with a question: and what about you?
"Ve'amrah: Amen" - her answer indicates that she too wishes to be a nezirah.
"Meifer es shelah" - the husband may annul her vow, because he never confirmed that he wants her to be a nezirah. He merely asked her a question, and she answered in the affirmative, and that does not constitute confirmation on his part.
"Veshelo kayam" - the annulment has no effect on his own nezirus, which remains fully in force.
The second case - the wife vows and the husband answers amen:
"Hareini nezirah, ve'atah" - the wife accepts nezirus upon herself, and asks her husband: and what about you?
"Ve'amar: Amen" - the husband answers that he too wishes to be bound by the vow.
"Eino yachol lehafer" - here he no longer has the power to annul her vow, since saying "amen" to her question of whether he wishes to be like her is considered a confirmation of her vow. Once confirmation has taken place, there is no longer any room for annulment.
In summary: in this mishnah we learned two cases of a husband and wife who take vows of nezirus. When the husband began and said "hareini nazir ve'at," and the wife answered "amen" - her vow remains subject to annulment, since the husband did not confirm her nezirus but only asked, while his own nezirus stands. But when the wife began and said "hareini nezirah ve'atah," and the husband answered "amen" - through his answer he confirmed her vow, and therefore he can no longer annul it.