区役所 （ くやくしょ, kuyakusho ）is your city hall in Tokyo

This is one of those practical but less than sexy words:  区役所　(city hall).  The ku part I guess indicates that you're in one of Tokyo's 23 wards. These pictures are of Toshima-ku's city hall.  If you look at the second picture, of the city hall sign, the first two kanj are Toshima, the last three kuyakusho.  City hall is where you get/renew your ID card, where you can register for health insurance, and where you can get information on a host of other things, e.g. where to go for community center Japanese classes (they might not actually have the information on hand but can tell you where to go/call), or where to find pretty much anything in your ward.  Tokyo city halls should have English speakers available to help you at least some days out of the week.