"Peri" is from the unfinished verb "prati" (the finished form being "oprati"; imperative form is "operi"). The "Peri" is in lieu of a suggestion as you should always do this, it's good for you, while "operi" would be the one time, in the moment "go brush your teeth." "prati/oprati" actually mean to wash, which is funny sounding to someone from an English background. Technically to brush is "četkati/očetkati" but that is about using the "četka" the brush to brush your hair. "Očisti zube" is possible but that is just "clean your teeth" as to use a toothpick or to floss.