Learn Spanish (Latin America): ¿Crees eso?

It comes from the regular verb creer (to believe). To say "you believe" informally, you take off the -er ending and add -es, making it crees.

Where is the word for "do" in this question?

Spanish doesn't use helper words like "do" or "does" to make questions. You simply use the regular verb form, like crees (you believe), and put question marks around it.

Is it okay to leave out the word for "you" (tú)?

Yes, it is very common! The -es ending on crees already tells us exactly who is doing the action. Spanish speakers usually drop the tú (you) unless they want to add extra emphasis.

Why use eso for "that" instead of ese or esa?

You use eso when you are talking about a general idea, a situation, or something you can't name. Since the speaker is asking about a general statement or thought (like "that thing you just said"), the neutral eso is the perfect word.