Hey there,
So I'll explain as far as I've learned so far, I'm just a new B2, so don't expect much from me :D
First of all, let me start by talking about the Subject and Object
Subject is a person or anything that does the action or does something in a sentence.
Object   is anything that something is being done to it

Remember this sentence: I love you.
Here "I" is the subject, because "I" is doing the action 

and "you" is the object, because something(love) is being done to "you"

Now!

Qu'est-ce qui & Qu'est-ce que both mean What, but when to use which?
Whenever the element (person, thing ...) is the subject! meaning it's doing the action, you use the qui (Qu'est-ce qui ...)
and whenever the element is the object and something is being done to it, you use que (Qu'est-ce que...)

An example of qui :

- Qu'est-ce qui fait ce bruit? (What is making that noise?) 

in the sentence above "What (qui)" is making the noise so it's doing an action, therefore you use qui

An example of que :

- Qu'est-ce que tu fais? (What are you doing?)
In this sentence "What" is not doing any action but You are doing the action, and the question simply asks for what you doing is? while in the previous example "qui" was making the noise!

It's a little bit tricky at first, but with practicing more examples you will get the hang of it.
A good way to know which one to use instantly is this:
Remember I said qui is the one that do the action? the subject one? then it always needs a verb or a reflexive pronoun (me, te ,se, nous, vous, leur, le, la, les ...) right after qui to do the action but with que we usually use a subject or something else after it and then the verb.

That's it, hope it helps.