The X-wing Sudoku method is for professionals. The task is to consider 2 parallel rows or 2 parallel columns. The 3x3 blocks are not used in this method.
Let's look at a concrete example of how to solve Sudoku using the X-Wing method. In the picture you can see 2 rows containing cells with the same number - a four.
Sudoku rules say that the number must be repeated 1 time in a horizontal and vertical line. In fact, our fours are lined up as if on a diagonal (visually forming the letter "X").
After that, you can safely cross out the fours from the other empty cells in the columns.
Once you have learned how to use the "X-wing" to solve Sudoku, you can move on to a more interesting strategy. It's called "Y-wing".