Fushūgaku is a Japanese word that means "compound words." It's like when you put two or more words together to make a new word that means something different.
For example, if I say "bookshelf," it means a shelf where you put your books. But if I say "Einstein-Rosen bridge," it means a way to travel through space and time like in Doctor Who or Star Trek.
Fushūgaku is important because it helps you save time and space by using fewer words to convey a complicated idea. Think of it like building with Legos. You can create lots of different things by stacking Legos together, and it's the same with fushūgaku.
But just like Legos, you have to know the rules to use them properly. You can't just put any words together and call it a compound word. There are specific ways to do it in Japanese, and it takes lots of practice and study to get it right.