A pronoun is a grammatical designation for words used to replace nouns and noun phrases. Some pronouns are easy to recognize: ella (she) is a pronoun that may be used instead of Marina or la chica de la calle Central (the girl from Central Street). Other pronouns are more difficult because they're really other parts of speech working as pronouns.
For example, compare Mucha gente cree que el castellano es difícil de aprender (Many people believe Spanish is difficult to learn) with Muchos lo creen (Many believe that). In the second example, muchos is an adjective that serves as a pronoun referring to gente and lo is a pronoun referring to que el castellano es difícil de aprender. Confused? Don't despair. This section will help you see how pronouns work.