Oh you mean Kohath, his descendants called Kohathites. ok
No, I mean Kohen. Aaron and his descendants were chosen from the Levites to perform the priestly service. (Exodus 28:1) Kohen, Kohanim(plural) is Hebrew for priest. Only the descendants of Aaron may perform the priestly duties such as, offer sacrifices on the altar, give the priestly blessing, redeem the firstborn.
Other descendants of Kohath, such as Korakh (Numbers 16:1) did not become priests.
If you had studied the Scriptures in the original language it would not be necessary to explain this.
Have you ever heard of someone with the last name "Cohen"? I'm sure you heard the whole thing about Kerry being Jewish (he's not, no matter what his grandparents were) because someone in his family had the last name Kohn, which is close enough.