The words mean:" Why" (why does the Mishna say that the woman who should have drunk but then her husband died needs Chalitzah and that Yibum is forbidden), "let also Yibum be allowed to be done" (since the Sotah's husband died without children - why should Yibum be forbidden?)

(Tisyabem Yibumi is the same verb written in double form.)

All the best,

Reuven Weiner