Objective:
Fill the 9x9 grid so that each column, each row, and each of the nine 3x3 blocks contains the digits from 1 to 9, only one time each (that is, exclusively).

Pencil Marks:
The usual approach to solve a Sudoku board is via pencil marks. They remind you which values are still possible for the current cell. You can toggle the pencil marks by clicking on them.