Difficult to understand; abstruse or unintelligible (because it is complex, technical, or obscure).
その論文は専門用語が多くて非常に難解だ。
That paper is full of technical terms and very difficult to understand.
彼の説明は難解で、結局何を言いたいのかわからなかった。
His explanation was abstruse, and in the end I couldn't tell what he wanted to say.
古典文学の一部は現代人にとって難解に感じられる。
Some classical literature can feel difficult to modern readers.