Theremin: the world's first electronic musical instrument, created in 1920 by the Russian physicist Lev Sergeyevich Termen.
Literal
Theremin: nineteen-twenty year, Russia [gen-の] physicist Lev Sergeyevich Theremin [subj-が] made world's first [gen-の] electronic musical-instrument.
A dictionary-entry caption with colon-introduction. The entire sentence is a single noun phrase modifying 電子楽器 'electronic musical instrument' via a long relative clause, with no main verb or copula — a classic encyclopedic fragment style. Note the ordering: date and creator first, the thing being defined last, all in a single nominal modifier chain. 一九二〇年 uses the kanji-character numerals (as opposed to the Western 1920), typical of formal/classical written contexts.