the useless knowledge that makes you unbeatable at exactly one thing.
means Bits of obscure, often pointless information — small facts valued less for their use than for the pleasure of knowing them.
from From Latin trivia, the plural of trivium, literally "a place where three roads meet" (tri-, three, plus via, road). Because crossroads were public, busy spots, the word came to suggest things common, ordinary, everyday. By the time it reached English, "trivial" meant trifling and unimportant — and "trivia" was revived in the 20th century as a name for those scraps of minor knowledge. (The medieval trivium, the lower three of the seven liberal arts — grammar, logic, rhetoric — shares the same root and likely reinforced the sense of "the basics.")