Why It is so hard to explain or show some thing

Why it is hard to explain something or learn something?
For example, when I first know the hadoop, I did a few search and made some little effort to try to figure out what it is. But I did not until I pay a lot of works to understand it.

Why sometimes some conception is hard to explain or learn?

Because:

1. The conception usually exist for solving or describing some problems.
2. The problem itself may hard for one to understand because one may never hit this kind of problem or this kind of problem does not exist in one`s world at all.

For example, hadoop describes a group of software,framework to solve the big data, distribution related problems.
The big data and distribution only occur for the big organizations who collecting or having very heavy business. It is hard for normal persons to imagine the situation not saying to know possible issues in that situation.

So even the hadoop docs shows you a paragraph of words say what it can do or what it is, you still do not understand. Because in the first place you do not know the problem.

So the correct way to learn something like hadoop is

1. know the problem to solve
2. know how this concept or product can solve that