Consider the following sentences:
I do what I want. Faccio quello che voglio.I saw him running down the street. L’ho visto che correva per la strada.This is the book I bought yesterday. I want the book about Michelangelo. I want a book about Michelangelo. He doesn’t study, which is a pity.

How do you use Che and Cui in Italian?

The relative pronoun cui is an invariable pronoun and has always a preposition before it. Therefore cui replaces che.

Relative Pronouns in Italian
La torta che hai cucinato è molto buona. Guarda la macchina che ho comprato. Questa è la casa che ha costruito mio padre.

Chi = who Che = that “Who” in English could be used meaning “that” too, in that case you need to use “che” in Italian. “Chi” only means “who” in questions.

What Che means?

Che is a Spanish diminutive interjection commonly used in Argentina. It is a form of colloquial slang used in a vocative sense as “friend”, and thus loosely corresponds to expressions such as “mate”, “pal”, “man”, “bro”, or “dude”, as used by various English speakers.

The relative pronouns “Che” and “Cui”

and: I medici, che hanno partecipato alla conferenza, erano americani. The doctors who attended the conference were Americans. By contrast, cui, a pronoun that means “which,” can take the place of an indirect object, an object preceded by a preposition.