It is a Sudoku app. When you select a difficulty level, it randomly presents a problem from a wide range of puzzles. Pressing the hint button will provide you with hints on how to solve it. The hints for the puzzles cover not only basic solving techniques but also various advanced strategies such as Naked Pairs and X-Wing, which are explained in the accompanying tutorial book.
In the Free version, once you clear level 4, you can play level 5, and after clearing level 5, you can proceed to level 6. By upgrading to the Pro version, you can always play up to level 9.