Microsoft's patent is for a keyboard with a small prism under each key. This prism has enough sides to reflect four different beams of light onto the key above it.
To display a character, the keyboard shines light in the shape of the character onto a prism. The beam reflects off of the prism onto the underside of a key, which then displays the character.
However, because these key markings are just beams of light, they can be changed on the fly. This gives them an advantage over keyboards with printed or etched keys, which stay that way for life.
(Read more at Makeuseof.com)