The passive voice consists of a conjugated form of the verb “to be” (am, was, will be, have been, had been) and a past participle.
Passive verbs are not wrong in themselves; indeed, some contexts (a scientific paper or a laboratory report, for example) require them. Problems arise when the passive voice obscures the implied subject of the sentence (the person doing the action of the verb), or when it disguises faulty or incomplete reasoning.
Generally, you should not use passive verbs; they are rarely necessary in critical essays. If you find yourself resorting to passive verbs, ask yourself if you are trying to cover up lack of knowledge; if the answer is yes, then you need to rethink your point and reformulate your sentence.
Wrong: Viola is given a ring. [Because the implied subject could be either Malvolio or Olivia, I cannot determine what point you are trying to make.]
Right: Olivia gives Viola a ring.
Right: Malvolio gives Viola Olivia’s ring.