It's used in a variety of situations, such as:
* Introducing a relative clause: "Le livre qu'il a lu est très intéressant." (The book that he read is very interesting.)
* After a verb of perception: "Je vois qu'il pleut." (I see that it's raining.)
* In certain expressions: "Qu'est-ce que tu veux ?" (What do you want?)
The apostrophe is used after "qu" when the following word starts with a vowel sound.