Relative pronouns (attieksmes vietniekvārdi) join a subordinate clause to the main clause. They use the same words as the interrogative pronouns: kas, kurš, and kāds.
In writing, the relative clause is separated by a comma.
Examples
Cilvēks, kas dzīvo šeit, ir mans kaimiņš.
The person who lives here is my neighbour.
Grāmata, kuru es lasu, ir interesanta.
The book that I am reading is interesting.