Normal sudoku rules apply. The cell with the gray square is even. Every 5 must have a 1 directly above it or a 9 directly below it. It may have both, but it doesn't need both. Digits in column 1 indicate the column in which the digit 1 appears in that row (e.g. if r4c1 is a 6, r4c6 is a 1). The same is true of column 5 and the digit 5, and column 9 and the digit 9. An X between two cells means that the digits in those cells must add up to 10.