All free icons are allowed to be used in non-commercial projects without permission.
Commercial projects: desired icon pack’s license can be bought, price depends from icons pack. Contact authors
Alternative for commercial projects: if you don’t want to buy license – you can add a link back in a very visible place with text “Icons by www.artdesigncat.com”
If you can’t find desired icon pack’s license or you have custom request please email to firstname.lastname@example.org