The X-wing Sudoku method is for the pros. The idea is to look at two parallel rows or two parallel columns. This method doesn't use the 3x3 blocks.
Let's look at a real-life example of how to solve Sudoku using the X-Wing method. As you can see in the picture, there are two rows with cells that have the same number—a four.
The Sudoku rules say that you have to repeat a number once in a horizontal and vertical line. As it turns out, our fours are lined up as if on a diagonal, which visually forms the letter "X".
Once you've done that, you can go ahead and cross out the fours from the other empty cells in the columns.
Once you've got the hang of using the "X-wing" to solve Sudoku, you can move on to a more interesting strategy. It's called the "Y-wing".