Translation:What is brighter than your face?
The meaning or the grammar?
Semantic meanings of sample sentences don't have to make sense. Understanding the grammar is what's most important. So, just take the question literally. (Someone's face is bright/shining, and someone else is asking what thing is brighter than the face.)
As for the grammar, here's a breakdown:
Apa = what, yang = that/which (used for emphasis), lebih = more, terang = bright, dari = than, wajah kamu = your face (literally "face (of) you").
In other words, a literal translation could be: "What is it that is more bright than the face of you?".
The "yang" is necessary in to point to the thing that is being asked about. Think of it literally as "What is the thing that is brighter than your face?" (i.e. includes "is") as opposed to "What more bright than your face?" Although in common speech it is often left out and is understood. But in proper form it is necessary IMO.
For those who do not understand the context:
"Wajah" means "countenance" while "muka" means "face". An Indonesian native speaker provided an excellent explanation here: https://hinative.com/ja/questions/181972
So, "Apa yang lebih terang dari wajah kamu?" implies that you look like the happiest person in the world.