What is the meaning of the words "Ba'im ba'Yamim"?
Ramban: In a person's youth, his days belong to him - they are called "Yamav" and and he is referred to as 'Omeid ba'Yamim'. However when he becomes old and his age exceeds that of his contemporaries, he is called 'Ba ba'Yamim' - like a man who enters a land that does not belong to him, as he travels from place to place, one day after the other.
What does it mean that Sarah ceased having "Orach ka'Nashim"?
Rashi: She ceased having Orach Nidus (the menstrual cycle). 1
Da'as Zekenim, Rosh and Riva ask that above (Pasuk 8), Rashi said that Sarah became a Nidah! Rather, Rashi means that she did not have Orach Nidos, i.e. menopause (cease to have menstrual cycles in her old age). Or, she returned to see after menopause, unlike others. R. Elyakim explains Rashi simply; she ceased becoming Nidah, and only today it returned to her. Even so, she laughed, for she did not consider this a sign that she can give birth - refer to 18:8:1:1*. See also K'li Yakar.