This game lets you practice writing words or kanji.
Even you're living in Japan and immersed in written Japanese all the time, you'll still probably get frustrated by how easy it is to forget. By far the best way to stop that happening is to learn how to write.
Amazingly, the part of your brain which handles writing is completely different from the part which handles reading. We know this because there have been cases where people lost the ability to read, but could still write!
Draw the stokes of the kanji which is being indicated by the '?', then tap the "Done" button when you think there aren't any more strokes.