Most people assume that Japanese tea is a thing, but it's not that simple. Japan has hundreds, or likely even thousands of different teas, all available in different regions and used for different occasions. If I was to pick out a common Japanese tea it would be matcha. Matcha is a finely ground green tea that is famously used for tea ceremonies. The tea is not steeped in the water, but is instead whipped into the water until it turns into a green froth. It's extremely bitter, and frankly one of the worst tasting teas I've ever sampled, and I've sampled it many times (I guess I'm a glutton for punishment). Matcha does however work well as a flavour, under some conditions.
The thing about matcha is that it is a very distinct and powerful flavour. When it's added to desserts, or in this case chocolate, it holds its own. Also, since many desserts and candies add sugar and/or other flavours, it takes down the strong bitter flavour and makes it tolerable. The key to a great matcha treat is balance, you want something that will cut the bitter matcha, but not kill the flavour.
In the case of this Kit Kat chocolate bar, the matcha is competing with the creamy white chocolate and the almost vanilla like flavour from the sakura (cherry blossom). I can't say that I've ever eaten sakura off of the tree, so I'm not really sure what it tastes like, but according to this bar it's a lot like vanilla. This flavour combination of the creamy chocolate, bitter matcha and sweet sakura is great, and the ratio of these flavours is very well balanced.
This is a great way to experience matcha, not very authentic compared to having a cup of match tea, but much tastier.