The two relative pronouns, che and cui, are invariable, which means that they do not change according to the subjects or objects they replace in the relative clause. We will cover each in turn.
As a relative pronoun, che has a variety of English equivalents. Che means that and which when it replaces things and who or whom when it replaces people.
Che can be used to replace a noun that functions as both subject or object. This distinction is not terribly important for you to think about while using che, but it can be interesting to consider as you focus on grammatical studies.