First, captions and subtitles are two similar systems that are often confused or used interchangeably. We are primarily speaking about captions in this post, but subtitles can be helpful as well.
Captions are bits of text overlaid on the video that are in the same language as the spoken-word content of the video. Captions are implemented or available when the viewer is expected to not be able to hear the audio. Subtitles work in the same way, but they are for translating spoken content into text in the viewers’ native language.
Captions can be “baked in” to the actual video file that is played so that they are visible to anyone who sees the video, but for many online video platforms, this is not necessary. YouTube, Vimeo, LinkedIn, and Facebook all have dynamic captions systems which will overlay captions over the video via the video player software on your device if the creator supplies captions data to go with the video file. This gives the viewer the option to turn on or off the captions depending on their needs or taste.