Subtitles can be necessary for opening up your content for those that are visually impaired. Aside from using titling in iMovie and the other video programs, we use two means to obtain subtitles for video files.
SRT files are the subtitle files that are used for video. This is a separate file that is overlayed onto the video. The .srt file is usually titles after the language it is written in (i.e. eng.srt). There is a program we recommend for writing an .srt file. It can be downloaded for Windows and OS X. You can find it here: https://aegisub.en.uptodown.com/