Abstruse things are difficult to understand because they are so deep and intellectually challenging. It might be hard to figure out how a toilet flushes but the technology that goes into making the Internet function is abstruse.
The Latin roots of the word abstruse are about concealing or hiding something, which is a good way to remember the meaning of this word. It is useful when describing something that is overly confusing, or if someone is deliberately making a story or a situation more complicated than necessary. It sounds and looks like obtuse, but abstruse is almost its opposite. Obtuse is dull or lacking a sharpness of intellect. While Abstruse is president of the chess club, Obtuse is hanging out by the parking lot smoking cigarettes.