The Gallery is displayed using the shortcode, allowing editors to place the shortcode where ever they wish the gallery to display.
The ID parameter allows the editor to indicate which gallery to display. This allows to display multiple and different galleries within a post.
Each gallery display will maintain its own layout and settings.
How to obtain the ID?
The best way to obtain an ID would be by looking at the url (in the browser) when editing a post.
How to use the ID?
Add the ID parameter inside the shortcode’s brackets, like this (using image example above):