This prequel set in the Bridgerton-verse tells the story of how young Queen Charlotte's marriage to King George sparked both a great love story and a societal shift, creating the world of the Ton inherited by the characters in Bridgerton.
Ton society was intensely class-conscious and the social hierarchy was rigid. Members of the ton came from the aristocracy, gentry, royalty and monarchs. Though some wealthier members of the middle classes could marry into the lower ranks of the gentry, such unions were not completely accepted by the elite ton.
... one could easily assume the word “ton” just means “town” — but, no. Like the slang terms “modiste” and “trousseau,” “ton” is borrowed from the French and then abridged. The original phrase is le bon ton, which directly translates to “good form” or “etiquette.” In this case, “good etiquette” means “good breeding,” aka the upper crust of high society.