A high key image is one that in its tonal range is dominated by lighter mid tones and white highlights.
This happens naturally if you a shooting where there is lots of light and/or bright surfaces to reflect the light. Or it can be achieved by over-exposing the image or post processing it in post to achieve the same effect.
If you study the histogram of a high-key image, you will see that it “tilts” very much to the right, with almost no blacks or shadows. The few black tones that you do see however, stand out so much more because of the “light” impression of the image overall.
In the example above you can see that the seagull and the black sails of the boat stand out, whereas the rest of the image is highlights or whites. This helps the subjects “pop”, i.e. they stand out because the contrast to the rest of the image is so significant.
High-key images have a light and positive feel to them, quite contrary to low-key images that in all aspects are the exact opposite.
Related reading
What is hard light vs soft light?
What is light falloff in photography?
