Relative pronouns connect a dependent clause and a main clause together in a sentence.  An antecedent is the noun or pronoun that the relative pronoun refers back to. The relative pronouns in English are that, what, which, whom, and whose. The relative pronouns in Italian are che, cui, il quale (and its forms), chi, quello che, quel che, and ciò che.

When the antecedent is a definite person, animal or thing, che, cui or a form of il quale is used.  Che is invariable and never used with a preposition.  Cui is also invariable, but it is always used with a preposition.  Il quale and its forms can be used with articles or articles plus prepositions.  It is mainly used in formal speech, writing and for clarity, and rarely in casual conversation.

La ragazza che vedi è mia sorella.  The girl whom you see is my sister.
Per le pillole di cui has bisogno ci vuole la ricetta.  The pills (of) which you need require a prescription.
Lei è la sola persona nella quale (or in cui) io abia fudicia.  You are the only person whom I trust.
È une medicina la quale (or che) non fa male allo stomaco.  It's medicine that doesn't upset your stomach.

When the antecedent is unknown or indefinite, chi is used when referring to people.  It is invariable and means "he/she who," "whoever," "the one who" and takes a verb in the third person singular form.  Quello che, quel che, and cìo che are all invariable and interchangeable.  They refer to things only and mean "what" or "that which."

Chi sta bene non va dal dottore.  He who feels well doesn't go to the doctor.
Chi trova un amico, trova un tresoro.  One who finds a friend, finds a treasure.
Non capisco quello che dice.  I don't understand what he's saying.
Cìo che scrivi è sbagliato.  What you're writing is wrong.