In the previous Mishnah we learned the position of the Sages, that a woman who became a deaf-mute - even though her marriage was a marriage by Torah law - her get is valid, since a get can be given to her by Torah law, because we do not need her consent. Mishnah 2 comes to bring support for this position.
The text of the Mishnah:
"He'id Rabbi Yochanan ben Gudgada al hacheireshes shehisi'ah aviha, shehi yotz'ah b'get" - Rabbi Yochanan ben Gudgada testified about a deaf-mute woman whose father married her off, that she is released with a get. And the explanation is as follows:
"Hacheireshes" - a woman who is a deaf-mute, even from birth.
"Shehisi'ah aviha" - her father married her off while she was a minor, and such a marriage takes effect by Torah law.
"Shehi yotz'ah b'get" - nevertheless, she receives her get while still a deaf-mute, even though her marriage was a marriage by Torah law - and this is because we do not need her consent.
"V'amru lo" - And they said to him:
With this the Sages answered Rabbi Yochanan ben Nuri, who asked in the previous Mishnah why the man is required to be of sound mind in order to give a get, while the woman is not required to be so. They said to him: "Af zu k'yotzei bo" - this too follows the very same principle. Just as a father betroths his daughter by Torah law and she is released with a get, so too a woman who was betrothed on her own with a betrothal valid by Torah law and afterwards became a deaf-mute can still receive her get.
In summary: This Mishnah brings the testimony of Rabbi Yochanan ben Gudgada about a deaf-mute woman whose father married her off, that she is released with a get, as support for the position of the Sages: even though the marriage was valid by Torah law, the woman's awareness and consent are not needed for the receipt of the get, and therefore her get is valid.