If you are a YouTube user (most probably) you should be familiar about YouTube captions, captions are transcripts of audio of a video, so if a person is unable to listen something still he can enjoy your audio by reading audio in text format.
All YouTube videos do not have captions because it depend on the person who uploaded the video.
Many educational videos have transcript as Google’s channel and many more channels provide transcript with videos.
Firefox has excellent add-on that let you download captions or transcript of a video. This add-on is useful because YouTube does not let you download captions or copy them.
You can download YouTube Captions Downloader add-on, it is very simple and small yet smart. After installing this add-on you will see downloading options for captions, see screen shot.
Go For YouTube Captions Downloader