This charm is "願かけ" which means to make a wish to a god. Some Japanese really like this kind of charm.

I was just there like 2 weeks ago!! I didn't buy one though... but from what I've heard it is mostly for illnesses such as headache. They keep the bad spirits away. Also, as some people have explained already they nowadays have 願かけ as well.

「願かけ」のお守りみたいですね。 「病気が治りますように」「恋がうまくいきますように」「仕事がうまくいきますように」「大学に受かりますように」など、いろいろのお願いができるお守りですね。