It is the secondary audio signal that is broadcast along with a television signal and its Primary audio.

Secondary Audio Programming (SAP) may be enabled through either the television, stereo VCR equipped to receive SAP signals, or an SAP receiver. SAPs may be used for a variety of enhanced programming, including providing a “video description” of a program’s Key visual elements, inserted in natural pauses, that describes actions not otherwise reflected in the dialog used by visually impaired viewers. This service also allows television stations to broadcast programs in a language other than English and may be used to receive weather information, or other forms of real-time information.