If you want the Captions to show automatically when the video starts playing, then set this to "Y" (yes).

When the video starts playing, the Captions are turned on by default. And if your viewer wants to turn it off, they can click on the "CC" (closed captions) Icon in the video player and turn it off.

So if this setting is set to "Yes", then captions are turned ON by default, and they can turn it OFF.

If it's not checked, then captions are turned OFF by default, and they can choose to turn it ON.