The playing field is a grid with 9x9 cells, divided into smaller grids with a side of 3 cells. Thus, the entire playing field consists of 81 cells. At the beginning of the game, there are already some numbers in them (from 1 to 9). The player is required to fill in the free cells with numbers from 1 to 9 so that in each row, in each column and in each small 3 ×3 square, each digit would occur only once.
There are multiple levels of difficulty you can choose. Beware of mistakes you can make.

Click on the cell you want to fill and then click on a number you want there. You can click on the "Notes" button to mark possible numbers without placing them.