In order to use an icon you must attribute it to its author, so we will be able to continue creating new graphic resources every day.
How to attribute it?
Insert the attribution on the page of the icon (for example in the page footer) or on the imprint page.
Paste this attribution on the final work so the authorship is known (for instance, in the acknowledgements chapter of a book)
Place the attribution on the credits/description page of the application.