This is the 2nd most frequent Italian word.
che
"Che" means "that," "which," "who," or "what," depending on context.
Che is used as an exclamation expressing admiration.
Che bello vedere il sole oggi!
How beautiful it is to see the sun today!
Che is used as a conjunction meaning 'that' in indirect speech.
So che domani sarà una bella giornata.
I know that tomorrow will be a beautiful day.
Che is used as a relative pronoun meaning 'that' relating to the noun 'book'.