Up until this point, the Mishnayot in Massekhet Yevamot have been teaching about cases where a husband dies with no children, but the commandment of levirate marriage cannot be performed because the dead man was married to a woman who was an erva – a close relative who is forbidden to marry – to the surviving brother. Thus such a woman cannot become a yevama, nor is there a need to free her to marry others through halitza.
Our Mishna teaches that there are other women whose status is such that they cannot marry the brother, and so, there is no possibility of yibum (levirate marriage). Nevertheless, halitza is still necessary so that she will be permitted to marry outside the family. The two categories of such women are issur mitzva and issur kedusha. The Mishna defines issur mitzva as women who are prohibited from marrying the potential yavam because of a Rabbinic ordinance forbidding their union, and issur kedusha as cases where they cannot get married because of a Biblical prohibition, but one that is less severe than an erva, like a kohen who is married to a divorcee or anyone who marries a mamzer – an illegitimate child born from a forbidden sexual relationship.
Abaye explains the terminology as follows:
Rabbinic ordinances are called issur mitzva because of the concept of mitzvh lishmo’a divrei hakhamim – that there is a mitzva to listen to the words of the Sages. According to the Nimukei Yosef, this mitzva is derived from the passage in Sefer (17:9-11) that teaches the importance of following the admonitions of the Sages.
Simple Biblical prohibitions are called issur kedusha based on the passage in Sefer (21:6-7) that refers to the kohanim as kedoshim – holy – in the context of forbidding them to marry a woman who is divorced.
The Rivan points out that aside from this pasuk that refers to a kohen, we also find a similar passage that refers to all Jewish people – vehitkadishtem veheyitem kedoshim (see 20:7) – which explains the source for issur kedusha in the context of anyone marrying a mamzer. In fact, according to some manuscripts of the Talmud, the passage that appears in the Gemara is kedoshim tihiyu (see 19:2), which applies to all Jewish people, not only to kohanim.