A pronoun is a word that replaces a noun. It stands for a person, place, thing, or idea that has usually already been mentioned. In a typical sentence, the sentence contains a subject (sujeito), a verb and an object (objeto). The subject and the object are usually nouns.
e.g. The cowboy fell off his horse.
Here the subject (cowboy) and object (horse) are both nouns. But if we are going to say several things about the cowboy, then we don't wish to keep repeating the word.
e.g. The cowboy drank so much whiskey that he fell of his horse.
So now he is a pronoun replacing the word cowboy.
The table below lists the more common pronouns.