The gameplay involves properly aligning a letter into its proper place in a specific word and font:
A random type-centric word with a missing letter appears at a variable point size. As the leading begins to shrink, you navigate the missing letter to the proper space and release its handle to lock it in place. The placement accuracy is measured and your score is calculated based on the size of the type, the leading height, and the perfection of placement—all measured in points. If you miss by too much and form an unnecessary ligature, one of your five ligature tokens will be lost. Lose all five ligatures and your game is finished. How many points will you Kern?
Points are accumulated based on how close to the proper spacing you get as well as the distance from which you lock in your answer. The video shows a good example of gameplay (music not included):
The game's reasonably fun to play even if you don't have typographical experience, but I imagine it will hold a special place the hearts of those who do.
App Store Link: KERN, $0.99