This is a kata renamed by Master Funakoshi, with its original name being Rohai. Translated to mean 'A brightly polished mirror', the name applies both to mirror imaging of many of the techniques, but more so, its philosophical significance is the idea that you should polish the kata through repeated practice so you gain an true understanding of the kata and of yourself. This can also be linked to the repeated use of the basic techniques of the kata, for although this kata appears very basic; its underlying principle is that through very basic training you attain skills that are truly brilliant.
This kata although looking simple uses techniques such as the jump, and simultaneous strike, which takes great skill and balance. Defence against stick attack is also covered here, with an interesting way of blocking the attack, like in Bassai Sho, and how to use the attackers weapon to your advantage.
This kata is not one of the more popular kata, but study of it is essential in order to improve, since it teaches you a great deal about the need for kihon study, with a philosophy that can apply to all of your karate.