This field is especially useful if you are creating a playlist with a number of videos or audios, and you want this specific list of files in this shortcode to last longer than what's configured in the global Settings page [as Link Expiry Time (in seconds)].
So if what you're creating is not a playlist, then you can ignore this field. Most will not need to use this field at all. So if you're not sure whether you need to use this, then that probably means that you don't need to worry about this, so ignore this.
If you leave this field blank, then the global expiration time will be used (which is usually set to 1000 seconds by default when the plugin is first installed).
For some reason, if you wish to override the global expiration time and set a custom expiration time for this specific shortcode, then you can set it here.
Enter the number of seconds here after which you want the media in this specific shortcode to expire.
If they click on a secure link, or start listening or watching a secure video, then it will not expire for that person. But after the expiration time, the media will stop working, so even if they pass it on to others, the link won't work.
So setting this field to 1000 means link will expire after 1000 seconds, which is 16.6 minutes.
Enter a number (eg., 5000).
Or you can leave this blank and the global expiration time will be used as the default.