Holiness (qāḏôš) is essential to the character of God. It means, to be set apart.
Wenham suggests that this phrase could be described as the motto of Leviticus.1 It is a reminder of the nation’s fundamental calling. The Hebrews were to conduct themselves in every sphere of life in such a way that they were set apart from the nations around them. Holiness is expressed in moral integrity, which is in turn symbolized by physical wholeness. In pursuing holiness, the nation would reflect the character of God.
2 Parle à toute l'assemblée des enfants d'Israël, et tu leur diras: Soyez saints, car je suis saint, moi, l'Eternel, votre Dieu.