Is there any subject which cannot be explained simply?
No.
What about quantum mechanics, organic chemistry, or rocket science? Surely these cannot be explained simply.
Any and every subject that can be explained logically, can also be explained simply.
So you are saying that even I can become an expert at quantum mechanics?
No. I am saying that every logical thing there is to learn in quantum mechanics can be explained simply. This holds for all subjects. However, that does not mean that every person can master every subject. Only people that master the basic building blocks of human knowledge can master any subject.
What are the basic building blocks of human knowledge?
First, the ones you learn early on: reading, writing, and arithmetic. Then, a few you are not forced to learn: probability, statistics, evolution and psychology.
Why do I have to learn probability, statistics, evolution and psychology?
Because these subjects explain 99% of what you see in the world. You need to learn probability and statistics to understand subjects like chemistry, physics, and engineering. You need to understand evolution and psychology to understand subjects like history, economics, government and religion. You need to know probability and statistics to understand these latter subjects as well. Thus, probability and statistics is as core to learning as reading, writing, and arithmetic.
I took a prob/stat course in high school. Is that good enough?
Probably not. After you took your reading and writing classes in elementary school, did you stop reading and writing or did you start practicing these skills everyday? You continued to use and practice them, right? Did you continue to practice your prob/stat skills? You should.
You're wrong. I've mastered probability and statistics, evolution, and psychology, and there are still subjects I can't find simple explanations for.
Do I need to master the building blocks to be successful in life?
No. But you need to know them if you want to be able to learn any subject.