Mishnah 4 in chapter 14 of Maseches Yevamos opens: "Shnei achim, echad cheresh v'echad pikeach, nesu'im lishtei achayos" - two brothers, one a deaf-mute and one of full capacity, who are married to two sisters, both of full capacity. We therefore have before us two marriage bonds of different kinds: the marriage of the deaf-mute is only rabbinic, since a deaf-mute can betroth a woman only by rabbinic enactment; whereas the marriage of the brother of full capacity to a woman of full capacity is a complete marriage from the Torah.
The first case - "Meis cheresh ba'al hapikachas":
If the deaf-mute who was married to the woman of full capacity died, his widow falls to yibum before his brother. The Mishnah asks: "Mah ya'aseh pikeach ba'al pikachas?" - what shall the brother of full capacity, who is married to a woman of full capacity, do? And it answers: "Teitzei mishum achos ishah" - she goes free as the sister of his wife. The surviving brother is married to her sister with a Torah-level marriage, and therefore the yevamah who falls before him is his wife's sister from the Torah - a full ervah. The obligation of yibum here is only rabbinic, since the marriage of the deaf-mute was rabbinic; and even if the obligation of yibum had been from the Torah, it would be pushed aside by the prohibition of ervah, which is from the Torah. Accordingly, there is no obligation of yibum here at all.
The second case - "Meis pikeach ba'al pikachas":
Here the problem arises. The brother of full capacity died, and his marriage to his wife of full capacity was from the Torah, so his widow falls to yibum before her yavam, the deaf-mute. "Mah ya'aseh cheresh ba'al pikachas?" - what shall the deaf-mute, who is married to a woman of full capacity, do?
We cannot say that she is exempt as the sister of his wife, since the deaf-mute's marriage bond with his wife is only rabbinic, and the yevamah is not his wife's sister from the Torah. On the other hand, his brother is his full brother in every respect, and the obligation of yibum with his brother's wife is from the Torah. It emerges that the zikah of yibum stands on a Torah level, while the prohibition of a wife's sister is only rabbinic - and a rabbinic ervah does not have the power to nullify a Torah obligation of yibum. Even so, his own marriage bond with his wife remains in force on the rabbinic level.
The Mishnah therefore establishes two halachos:
"Motzi ishto b'get" - the deaf-mute must divorce his wife. The zikah that falls before him originates in a marriage that was from the Torah, and it turns out that his present wife is the sister of the woman bound to him. For this reason he cannot remain married to her, and he must send her out with a get.
"V'eishes achiv asurah l'olam" - the widow remains forbidden forever: he cannot perform chalitzah with her, since he is a deaf-mute; and he cannot perform yibum with her, since she is his wife's sister on the rabbinic level, and she is a rabbinic ervah.
In summary: in this Mishnah we learned how the two levels of marriage - Torah-level and rabbinic - determine the halachah. When the deaf-mute died, the yevamah is forbidden and goes free as the sister of his wife, because a Torah-level ervah overrides a rabbinic obligation of yibum. And when the brother of full capacity died, a rabbinic ervah does not override a Torah-level zikah of yibum, and therefore the deaf-mute sends his wife out with a get because she is the sister of the woman bound to him, while his brother's wife remains forbidden forever - since he is not capable of chalitzah and he cannot perform yibum with her.