Closed Captioning is a term used to inform that a video has some other information (in form of captions) that the user can view by activating them (by clicking or so). A video with Closed Captioning can be identified with the “CC” icon at the top or bottom of the video.
This term is widely used in Television, IP video, Internet video.
According to Wikipedia:
Closed captioning is a term describing several systems developed to display text on a television or video screen to provide additional or interpretive information to viewers who wish to access it. Closed captions typically display a transcription of the audio portion of a program as it occurs (either verbatimor in edited form), sometimes including non-speech elements.
The term "closed" in closed captioning indicates that not all viewers see the captions—only those who choose to decode or activate them. This distinguishes from "open captions" (sometimes called "burned-in" or "hardcoded" captions), which are visible to all viewers.
Watch a video to understand what is closed captioning:
Read here : On television, how does closed captioning work?
According to fcc.gov
Closed captioning allows persons with hearing disabilities to have access to television programming by displaying the audio portion of a television program as text on the television screen.
Read a good Article describing what are Captions in reference of videos:
Web Captioning Overview
Captions are text versions of the spoken word. Captions allow the content of web audio and video to be accessible to those who do not have access to audio. Though captioning is primarily intended for those who cannot hear the audio, it has also been found to help those that can hear audio content and those who may not be fluent in the language in which the audio is presented….
Caption as seen on DVD
Caption as Seen in Media Player on Computer