Clauses introduced by a relative pronoun, e.g que, el cual, quien... (that, who...) are called "oraciones relativas". The relative pronoun will be referring to a reference mentioned before that could be something or someone, and will give extra information about this reference.
Here are some examples:
El perro, que jugaba en el parque, no paraba de ladrar.
The dog, that was playing in the park, wouldn't stop barking.
La maestra, quien había regañado a los niños antes, estaba muy enfadada.
The teacher, who had told the children off before, was very cross.
El chico que me está mirando me está poniendo nerviosa.
They boy who is staring at me is making me nervous.