The commandment of "You shall be holy" is a positive commandment. Many commentators, including the Rambam, hold that the purpose of this commandment is to make the laws of forbidden sexual relationships (עריות) included in a positive commandment. The Ramban, however, does not count this Positive commandment in the Rambam's count in his Sefer Hamitzvot. Ramban in his commentary to the Torah explains this, by saying that the Torah previously gave us prohibitions in regarding food and sexual relations. The Torah by stating a broad principle קדושים תהיו "be holy", is telling us to sanctify ourself with that which is permitted as well as staying away from that which is prohibited. The Ramban explains that the Torah was worried that one might overindulge in even the permitted areas. Therefore, the Torah gave a general commandment to keep oneself holy by refraining from that which is prohibited and to even refraining from overdoing that which is permitted. The Ramban here introduces a celebrated phrase נבל ברשות התורה, a scoundrel who operates within the technically permitted Torah rules. Ramban writes that this pasuk forbids inappropriate behavior even if no technical violation of the Torah law is committed.
Rashi, however, explains this commandment in a different way. The word Kedoshim here comes to tell us to keep away from that which is prohibited only, namely עריות. Once one is far from עריות, there is Kedusha.
The Ketav VeHakabala says in regard to the concept of Kedusha, one who forbids himself from doing that which is permitted is regarded as a Chassid and not a Kadosh. He explains that being Kadosh is not only in a spiritual sense but also in physical activities that one engages in. This means that one should not always be struggling to find pleasure in his actions but to understand that he is observing the commandments of his creator.
Abarbanel says, also in regard to Kedusha, that this Parsha repeats many commandments that were already listed earlier. This was done to stress the importance of performance and awareness of Kedusha. So important are these commandments, says Rashi, that this Parsha was recited בהקהל, in the presence of all the people of Bnai Yisrael. By publicly reciting the laws of Kedusha, the laws are guaranteed to be observed for generations to come. By observing these laws we become a holy nation.