What
Captions, scripts, and transcripts all provide text content for videos. Captions appear on the screen while the video plays and follow along with the dialogue or audio. A script is a document that accompanies a video file and provides contextualized dialogue, while a transcript marks the script with speakers and timestamps.
Why
Providing an alternative to audio makes videos usable for viewers who are D/deaf or hard of hearing. It also enables access to content for those who have a slow internet connection, do not have headphones or speakers, or are in a noisy location.