EXIF data: It’s the unsung hero of iPhoneography, and digital photography in general. It’s where location data is stored so you can see your pictures on a map. It remembers when you took that photo, what ISO you used, along with all kinds of other handy data (focal length, flash on or off, even the white balance setting you used).
But on iOS, the EXIF data is mostly hidden from you. But with EXIF-fi, you can not only read it but edit it.