The phrase "approved helmet" means a helmet that has been tested and certified as safe and suitable for protection by an official organization or authority. Full definition
Always wearing approved helmets properly, while also advising that helmets can not prevent all injuries; (blg.com)
Correction: An earlier version of this article mischaracterized National Operating Committee on Standards for Athletic Equipment's role in approving helmets for use in the NFL and NCAA. (inc.com)