Sholom Rav,

Ma'amar is the Kidushin that the Rabanan instituted regarding a Yevamah. Min he'Torah a Yavam acquires his Yevamah just with Bi'ah (intimacy). But the Rabanan instituted that, just as a man marries a woman only after he has betrothed her, so too, should a Yavam first betroth the Yevamah before performing Bi'ah.

Be'Virchas kol tuv

Eliezer Chrysler.