If you look at the conjugation tables I mention in another comment you'll notice two possiblities for the "you" imperative, one for "você" (corra) and the other for "tu" (corre), but with a negative command the "tu" version changes to "corras". The sentence "Não corras por favor" should be accepted.
They are just different forms (conjugations) of the verb "correr" or "to run" in English. You can find all the situations where they could be used by consulting tables of conjugations like these:
In the sentence above "não corra" is being used to tell (order) an individual not to run (look under "Imperativo negativo" in the tables). If you wanted to tell a group of people not to run you would use: "Não corram por favor".