“It shall be brought in water, and will be impure until the evening, when it will become pure!” (Vayikra 11:32)
It is a negative commandment that a Kohein who has purified himself on that day may not serve in the Temple. This means that a Kohein who has immersed himself in a Mikveh (a bath for ritual purification) because he was impure, may not serve in the Temple until sundown. This is true even though after immersion he is not considered as impure as he had been before, as before immersion he was considered a primary level of impurity, whereas afterwards he is only a secondary level of impurity. Nevertheless, he may not serve in the Temple.
Amongst the roots of the commandment are that the Kohein is a messenger interacting between the people of Klal Yisrael and their Father in heaven, and it is through his acts and offerings that a person is accepted by his Creator and his sins are atoned for. It is therefore incumbent upon him to be at the zenith of purity at the time of his service, and Hashem knows when is the time of the ideal level of purity, and when he may serve in the Temple.
This commandment applies to Kohanim, when the Temple was standing. One who transgresses this commandment and serves in the Temple when impure, is liable for death by the act of heaven.