What is a subtitling service?

Subtitling is adding text to any audio-visual medium (corporate, promotional video, television, and film) to deliver the message being spoken.
There are various reasons for choosing subtitles while watching the media. Irrespective of whether a person is hard of hearing or deaf, the other reasons are:
There is a loud sound while viewing the media.
Viewing media in an overseas language
Watching delicate subject matter media in public.
Taking notes or watching educational videos.