The continuation of the passage in Sefer Devarim (18:3) that obligates the butcher to offer the zero’a (foreleg), leḥayayim (jaw) and kevah (maw) to a kohen, instructs the individual who is shearing his flock to offer some of those shearings to the kohen, as well (see Devarim 18:4). In concert with the Torah’s commandments, the eleventh perek of Massekhet Ḥullin that begins on today’s daf follows the discussion of the zero’a, leḥayayim and kevah with Perek Reishit HaGez “the first shearings,” focusing on this law.
The Shulḥan Arukh (Yoreh De’a 333:11) rules that ideally the wool should be given at the very beginning of the shearing, although if it was not performed at the very beginning it could be done in the middle, or even in the end. Some suggest that the wool should be given from the very first animal that is sheared (as the Netziv writes in his Ha’amek Davar), or alternatively, if many animals are to be sheared it should not be left until the end, but should be done at the beginning, i.e. after five animals are sheared.
As the Mishna points out, this law applies only to sheep, and although the Torah does not obligate that a certain amount be given, the Sages did establish a minimum of five sela in Judea.
One explanation offered for this mitzva is suggested by the Rambam (see Mishneh Torah, Hilkhot Bikkurim 10:5) who argues that since kohanim did not receive a share in the land of Israel, the Torah obligates the community to concern itself with their welfare. Thus we find that they receive teruma in the form of bread and wine as well as meat when they are given the zero’a, leḥayayim and kevah and eat of Temple sacrifices. Aside from sustenance, they are supposed to be clothed by means of these gifts, and Reishit HaGez offers that to them.