The direct object is the item in the sentence that receives the action of the verb.  A direct object pronoun replaces the direct object in Spanish. See the example below:

• Susana tira la pelota. [Susana throws the ball.]
  • Direct object:  la pelota
  • The direct object pronoun which could be used to replace la pelota would be: la (because la pelota is feminine).

So instead of saying Susana tira la pelota, you could say:  Susana la tira. [Susana throws it.]

Unlike in English, in Spanish the direct object pronoun goes IN FRONT of the verb. So in the example above, Susana la tira, you are literally saying “Susana it throws.”

If the direct object you are replacing is masculine, you would use lo for the direct object pronoun. So for example:

• Susana tira el libro. [Susana throws the book.] Here el libro is the direct object.
• Susana lo tira. [Susana throws it.]  Here lo is the direct object pronoun referring to el libro. 

If the direct object you are replacing is masculine plural, you would use los as your direct object pronoun. If it is feminine plural, you would use las.

 

(source)