Closed
Closed captions are when the captions are in a text file format which you can turn off in the video player. YouTube, Facebook, and Vimeo all support closed captions. As do Amazon Video, Netflix, Disney Plus and more. Almost all video players support the SRT caption file format, so it's the most frequently sought file for closed captions.
Open
In contrast, open captions are burnt-in, i.e. encoded into the video file itself and cannot be turned off. Open captions are frequently used in business communications because the video player may not allow users to turn the captions on/off. There is also the possibility users may not realise captions are available. It's safer to use open captions and burn them in.