High Key simply means that most of the tones within the image are light.   

This was a popular style for portraits with white backgrounds and light toned faces in black & white, but it is equally suitable for colour images too.

The effect can be achieved by having a suitable image eg light / white background :

then exposing so the subject is very much in the lighter range of tones

using lighting to achieve the effect

or using postwork to achieve the same effect.

The example is a Passion flower.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html