A " crash helmet" is something that someone riding a bicycle or a motorcycle might have on their head to protect it in case they fell off of the bicycle or the motorcycle and hit the ground.
Through the glasses Pete could see the jeeps, filled with men in their gleaming grey uniforms, crash helmets tight about their heads, blasters glistening in the pale light.
A " crash helmet" is something you wear over your head, like a motorcyclist would wear, to protect your head in case you fall out of the vehicle and hit your head.