A high key image is one which consists almost entirely of bright tones. This works really well for subjects with white or near-white tones and gives a very bright, airy look. Not every image needs a full range of tones from solid black to brilliant white, and not every image needs the ‘perfect’ histogram. Histograms are there to tell you what’s happening, not what to do.
A photo where the tones are predominantly bright or white. It’s partly the subject that makes a photograph high key – a white cat on a white cushion, for example, and partly the exposure technique – slight overexposure will give a high key look.