He is talking about something specialized that a commoner cannot understand.

He ate one apple.

Abstract nouns, unlike nouns that refer to concrete things, refer to concepts or states that do not have a specific shape or color.

His artwork is very abstract, making it difficult to understand.

He has a deep interest in abstract art.

His room is decorated with colorful abstract paintings.

He is passionate about researching pure mathematics.

He is majoring in applied mathematics at university.

He is a world-famous mathematician.

Combinatorics is a branch of mathematics that studies combinations of things.