The book of Vayikra describes in great detail the different precepts related to the service of the kohanim - priests - in the Mishkan - tabernacle - in the desert and, later, in the Temple of Jerusalem.
Hence, this book is known in rabbinic literature as "Torat Cohanim" - the Torah of the priests - because it is dedicated almost exclusively to the laws that concern them.
Today, we cannot fulfill most of these commandments because we do not have a Temple, a High Priest, and so on. However, our sages explain that studying these laws is the way to observe them today. Even though we are not in the place where the commandment was meant to be fulfilled, nor is it the appropriate time for it, since the Torah transcends time and space, we can, through its study, still fulfill these commandments (this idea applies to all 613 commandments, so we can say without any difficulty that a person fulfills them all even today).