Explanation
Description
noun, verbal noun, の adjective
When you make your theory bright (and clear and easy to understand) you must have come up with a great explanation for it. Before, it was dark and murky in everyone's mind. But, when you gave your explanation, it became bright and easy to understand! Yay!
説明 is a general, everyday word that is used when explaining something unknown or unfamiliar to make it understandable. You may see it when explaining how to use a device or describing a situation, for example.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
彼は説明が下手です。
He is not very good at explaining.
このボードゲームのルールを説明してもらえますか？
Could you explain the rules of this board game?
電車のスピードを急に落とした理由を説明してください。
Please explain why you suddenly slowed down the train.