You must call this when done with the library, to free internal resources. It is safe to call this when the library isn't initialized, as it will just return immediately.
Once you have as many quit calls as you have had successful calls to TTF_Init, the library will actually deinitialize.
Please note that this does not automatically close any fonts that are still open at the time of deinitialization, and it is possibly not safe to close them afterwards, as parts of the library will no longer be initialized to deal with it. A well-written program should call TTF_CloseFont() on any open fonts before calling this function!
This function is available since SDL_ttf 2.0.12.