No need to explain the game which is known to the majority of people. The rules are simple. Add the missing numbers to the grid and complete the puzzle.
From the first sight this game could look boring for you as there is only one simple grid with the numbers in chaotic order. But don't judge it only for the look because when you start playing everything will become different. Your brain will start working and if you solve the puzzle the satisfaction you get will be just great. Try Sudoku and see if you can complete the puzzle!