Bamidbar 18:19:
כֹּל תְּרוּמֹת הַקֳּדָשִׁים אֲשֶׁר יָרִימוּ בְנֵי יִשְׂרָאֵל לַיהֹוָה נָתַתִּי לְךָ
All the gifts of the holy [offerings] which are set aside by the children of Israel for the Lord I have given to
There were 24 mishmaros, “guards,” of kohanim who rotated to perform the priestly duties in the Beis Hamikdash on a weekly basis. Each mishmar would serve for two weeks a year, and within each weekly rotation, any individual kohen would serve for one day. This means that the kohen would perform priestly duties for only two days per year. Yet, the Torah specifies 24 matnos kahunah, priestly gifts, that each member of Bnei Yisrael had to provide. These include terumah, terumas ma’aser, challah, and pidyon haben; twenty-four sources of income for two days’ work!
In fact, however, the kohen’s main function was not to serve in the Beis Hamikdash, but to teach Torah: For a priest’s lips shall guard knowledge, and teaching should be sought from his mouth, for he is a messenger of the Lord of Hosts (Mal. 2:7). His permanent daily job was to instruct the people, to teach the community. The book of Leviticus is known as Toras Kohanim, not because it deals with the laws surrounding the priesthood per se. The book of Leviticus primarily deals with laws pertaining to Bnei Yisrael, not only the priesthood. It is called Toras Kohanim because the Kohanim were to teach these laws to others. (Boston, 1977)
