Learn French: Qu'est-ce que tu trouves dans le sac ?

It is made of two parts: que (what) and est-ce que (the standard phrase used to turn a statement into a question). Because est-ce que starts with a vowel, the e in que is dropped, leaving qu'. Literally, it means "what is it that", but in everyday English, it simply means "what do" or "what are".

Could I just say "Que tu trouves dans le sac ?" instead?

No, you cannot put que (what) right next to a subject like tu to form a question. When you want to ask "what" at the very beginning of a sentence using this question style, you must use the full chunk qu'est-ce que.

How do you pronounce qu'est-ce que?

Even though it looks long and complicated, it is pronounced smoothly as just two syllables: "kess kuh".

Why does trouves end in an "s"?

Because the subject is tu (you). For regular verbs that end in -er like trouver (to find), the tu form always gets an -s at the end. Remember that this -s is completely silent!