Captions allow people who can't hear a video's soundtrack to have access to a text version of the information provided in the audio.
If you decide to caption your own video content rather than outsource this job to a captioning service, make sure the captions provide an accurate and meaningful alternative to the audio. In particular, when writing captions for audio content in a video, make sure all the spoken content is available in captions, as well as indications of speaker switching (using ">> SPEAKER NAME:") and descriptions of significant non-verbal sounds (setting them off in square brackets, such as "[BUTTON CLICK]").
Video: Writing closed captions