Swordfish is a generalization of X-Wing. If a digit is a candidate for at most three cells in each of three different rows and the positions of those cells fall into the exactly three columns, then this digit cannot be a candidate for any other rows in the same three columns.
Consider the following Sudoku puzzle:
In the second, fifth and ninth rows of this 9x9 Sudoku puzzle, the digit 4 is the candidate for the last three columns (in those yellow cells) and does not appear in any other columns of those three rows. This is a Swordfish pattern. Now the digit 4 can be safely eliminated in those blue cells that share the common columns with those yellow cells.