You Shall Sanctify Him The Kohain

You shall sanctify him (the Kohain), for he offers the food of your G-d. (21:6) The second pasuk is describing the Mitzvah of sanctifying a Kohain. One must treat the Kohain with respect - honor him - he goes first etc. But this obligation upon Benai Yisroel only comes after the first pasuk.

First the Kohain must be holy to Hashem. The kohain must first sanctify himself with his own actions. Only then is there an obligation for Benai Yisroel to treat him with Kedusha. (Oznayim LaTorah)