You can make your videos more accessible to viewers by adding subtitles. To add subtitles, you will need a VTT file. You can convert from another format if you do a Google search for VTT convert.
Navigate to the video you want to add subtitles for under Content > Videos and click Edit. Then scroll down and click Upload Subtitles.
If you need to know how to create subtitles in the VTT format, try a free service like VTT Creator.