noun, verbal noun, の adjective
When you make your theory bright (and clear and easy to understand) you must have come up with an explanation for it. Before it was dark and murky in everyone's mind. But, when you give your explanation, it gets bright and easy to understand! Yay!
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.
Please explain why you suddenly slowed down the train.
How could he be fit to be company president with a terrible explanation like that?!
I’m sorry, I don’t understand at all. What does your explanation mean?