Sudoku is a puzzle based on a small number of very simple rules:

Every square has to contain a single number
Only the numbers from 1 through to 9 can be used
Each 3×3 box can only contain each number from 1 to 9 once
Each vertical column can only contain each number from 1 to 9 once
Each horizontal row can only contain each number from 1 to 9 once
Once the puzzle is solved, this means that every row, column, and 3×3 box will contain every number from 1 to 9 exactly once.

In other words, no number can be repeated in any 3×3 box, row, or column.