Que is used to replace a direct object whether the direct object is a person or an object.
For example:
J’ai acheté une voiture. La voiture est de couleur rouge. I bought a car. The car is red.
To combine the two sentences and hence avoid the repetition of the word “voiture” which has the function of a direct object in this sentence, we use the relative pronoun “que”:
La voiture que j’ai achetée est de couleur rouge. The car that I bought is red.
Note that we added “e” to the past participle because the direct object preceded the verb. To learn more about this, review the lesson “Object/Verb agreement with the auxiliary “avoir” under the folder “ The Passé Composé” in our Grammar Catalog.