From Ten Condiments for the Yetzer Hara (#E-90) by Rabbi Avigdor Miller
What does it mean “Kedoshim tihyu —You shall be holy?” In pashtus (simply put), it means you should be as much in resemblance to Hakodosh Boruch Hu as you could be. “Ki kadosh ani Hashem — Because I, Hashem, am Holy.” You should try to resemble Me as much as possible. Right now that seems farfetched. Seems like that’s only for tzadikim gemurim (the totally righteous). Pay attention: It’s actually down to earth. It’s a very simple, clear thing.
Holiness means full mastery over your desires
Kedoshim tihyu means learn self-control. The sechel (mind) has to control all of your acts, not your desires. “Lo sasuru acharei levavchem vaacharei eneichem” (Shema): don’t follow what your eyes see; or what your mind thinks. Follow only what the sechel tells you. Even though there might be kosher indulgence. Let’s say you have a whole basement full of kosher shnaps; brandy, all kinds. Forget about it. Keep it away. Don’t go for taavos (physical desires).
Kedoshim means a person should be in full mastery over his desires, which is such a general rule that it includes everything. A person must be able to control his mouth and not talk. About Hakodosh Boruch Hu it says, “Yosheiv besaiser elyon — He sits above in silence.” Hashem doesn’t talk. If He talked, the whole world would be thundering every minute. Hashem keeps quiet: besaiser. So we emulate Him and keep quiet. (Unless you’re saying Torah. Otherwise don’t talk). “Kedoshim tihyu” means “prushim tihiyu” — be abstentious. In every area of life, learn self-control.