El libro que leo es interesante.
English: The book that I am reading is interesting.
In Spanish, 'oraciones subordinadas adjetivas' are relative clauses. They are sentences that describe or give more information about a noun, just like adjectives.
Use these clauses in Spanish when you want to add information about a person, thing, or place mentioned in the main sentence. They help specify or identify which person or thing you are talking about.
El libro que leo es interesante.
English: The book that I am reading is interesting.
La chica que vive aquí es mi amiga.
English: The girl who lives here is my friend.
Conozco a una persona que habla español.
English: I know a person who speaks Spanish.
El hombre cuyo coche es rojo es mi vecino.
English: The man whose car is red is my neighbor.