To learn how captioning works, we recommend that you read the following article: Wikipedia - Closed Captioning
An estimated 24 million Americans have enough of a hearing loss that they cannot fully understand the meaning of a television program. This is especially true of the elderly, the fastest growing category of individuals who are deaf and hard of hearing.
Captions enable viewers who are deaf and hard of hearing to participate with family and friends in America's favorite pastime: watching TV. Captions can also benefit adults and children learning to read, as well as people learning English as a second language.
Like subtitles, captions display spoken dialogue as printed words on the television screen. Unlike subtitles, captions are specifically designed for viewers who are deaf and hard of hearing. Captions are carefully placed to identify speakers, on- and offscreen sound effects, music, and laughter.
Closed captions are hidden as data within the television signal, and they must be decoded in order to be displayed on your TV screen. With either a set-top decoder or a caption-ready TV set, you can switch captions on or off with the touch of a button.