This is the 1414th most frequent Russian word.
главного
"Главного" means "main" or "chief" in English.
The word "главного" is used here in its function as an adjective in the genitive case, agreeing with "героя" (character).
Главного героя представили в начале книги.
The main character was introduced at the beginning of the book.
Here, "главного" is used as a noun, referring to the main candidate, due to the implied context where the noun is omitted.
Мы выбрали главного среди кандидатов.
We chose the main one among the candidates.
In this example, "главного" is a short form noun derived from "главный" (chief) to mean supervisor or boss in informal speech.