A useful feature of edX is its ability to display a synchronized transcript as your video plays. This helps students follow along with what you're saying, especially if you are explaining complex concepts and/or if a student's first language is not English.

One unique benefit of including a transcript is that it scrolls automatically while the video plays within the edX platform; if a student clicks on a word within the transcript, the video will rewind or fast-forward to that word (or phrase) in the video. Instructors can also allow students to download the transcript in different formats including a .pdf or .srt file for reading offline.

Have a look at an example of the transcript from edX101 How to Create an edX Course :

For the transcript to automatically play in sync with your course video, it must be a .srt file. EdX will detect whether or not a YouTube video you've added to a course includes a transcript or not, as illustrated in the following screenshots...