X-Wing is a more advanced strategy that does not appear until the "Tough" volumes in my collection.
In the Sudoku puzzle below, the only squares in rows one and eight that can contain a 9 are highlighted. As the X shows, if a 9 occurs in the upper left, it must also occur in the lower right, and vice versa. It follows that these same cells must contain the 9s in column-5 and column-8.
As a result, 9 can be eliminated from the other squares in those columns – you can eliminate the dashed 9s in rows 2 and 9.
This is an easy pattern to understand, but in my experience, it does not occur that frequently, as compared to XY-Wing, which is coming up!