pronoun: replaces something

avoids repetition in sentences

allows compound sentences

Ejemplo:

La clase es interesante. La clase es a las doce.

The class is interesting. The class is at 12:00.

Using relative pronoun

La clase que es a las doce es interesante.

The class that is at noon is interesting.

OR

La clase que es interesante es a las doce.

The class that is interesting is at noon.