"Х-wing" is an advanced sudoku technique, which is based on the two parallel rows or two parallel columns. You shouldn't pay attention to the 3x3 blocks as they aren't involved in this strategy.
It will be easier to understand this technique if you look at the example.
Let's take a look at the two rows. There are two cells in each of them that contain a note of 4. Since 4s can't repeat in the same row or column, we can safely assume that 4s will be placed diagonally – either in light blue cells or dark blue cells.
Now let's zoom out and take a look at the columns involved. Since 4s are diagonal, there will already be one number 4 in each of these columns. That means that we can't write it again.
So, we can confidently remove 4 from all the remaining notes of these two columns.
Now you know how to apply X-wing technique in Sudoku and can proceed with the following advanced Sudoku strategy "Y-Wing".