If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
The goal of Sudoku is to fill in the 9x9 grid with the numbers 1-9. However, a number can only appear once in each row, column, and 3x3 box. To enter values, click the square you know the value for and use the number pad to enter numbers 1-9.
Play with your mouse.