A Swordfish is the 3-row extension of an X-Wing. A candidate appears in at most three cells in each of three rows, and those cells span exactly three columns. The candidate can then be eliminated from all other cells in those three columns.
After checking for X-Wings, look for a candidate that appears in two or three positions in each of three rows, with all positions falling in the same three columns.
Each of the three rows must contain the candidate once. With positions limited to three columns, the three placements fill all three columns. The candidate cannot appear elsewhere in those columns.
Let's find a Swordfish. We focus on digit 6 and look for three rows where 6 appears in positions spanning exactly three columns.