The very first Pasuk of this week's Parsha tells us of the Mitzvah of "Kedoshim Tihyu," "You shall be sanctified." Both Rav Sa'adya Gaon and Rashbatz, 2 of the earliest enumerators of the 613 Mitzvot in the Torah, both include Kedoshim Tihyu as a separate commandment. But this phrase is quite vague; what exactly does it mean to "be sanctified?"
The two most famous explanations are those of Rashi and Ramban. Rashi explains, that Kedoshim Tihyu is a requirement to create a "fence around immorality." This command has been upheld by Chazal in several forms (Yichud, Kol Ishah, etc.).
Ramban's interpretation is that the Pasuk instructs us to avoid the common mistake of allowing something because it is technically permitted, even if it is against the "spirit" of the Torah (Shelo Yehai Naval Birishus Hatorah). Gluttony, Nivul Peh, alcoholism, and other such actions are not technically prohibited, but are clearly not the Torah way. The Ramban explains that "Kedoshim Tihyu" teaches us to avoid such actions.