"The way popular media matures is by moving away from binary 'good vs evil' tropes and embracing the moral gray areas that define real adult life." [1]
It might describe a shift in a franchise (like a gritty reboot of a cartoon) where the "entertainment content" is being aged up or "matured" for a legacy fanbase.
It often refers to how a piece of media handles adult themes (like violence or complex social issues) within a format typically aimed at younger or general audiences.