How it works
When all candidates for a specific digit in 2 rows are contained in only 2 columns, it will also complete those 2 columns. This allows you to eliminate all the candidates for that digit in the remaining cells in the columns. The rows or columns however can not be located in the same chute.
This diagram shows how it works. The digit represented by X is present in 2 cells in rows 2 and 8. Both candidates are in columns 2 and 8. As there are only 2 ways to complete those rows, both columns will be covered by the complement and all the candidates for the digit in the columns can be removed (marked with red).
It is the same if the digit in 2 columns is contained in only 2 rows, as it would be if the Sudoku was rotated 90 degrees.
Examples
In this example there is an X-Wing located on row 4 and 7. On those rows, the digit 2 is only present in column 2 and 9. The possible candidates have been marked with 2 different colors, either both candidates in green, or both in blue must be placed. This rules out the possible candidate in cell r9c9, which can be eliminated (marked with red).