The phrase "to wear a helmet" means to put a protective headgear on your head to keep it safe.
Sentences with «to wear a helmet»
I can get a ticket for not wearing a helmet while riding a bicycle in my city. (religion.blogs.cnn.com)
Write things students can do to stay safe while playing sports, such as wearing a helmet while riding a bike, following the rules in a game, etc.. (educationworld.com)
Instead, many of the top grossing movies show characters who don't wear a helmet when riding a motorcycle or horse, etc.. (verywellfamily.com)