A Swordfish is an X-Wing one size larger. Take three rows in which a digit has two or three candidate cells each, and where all of those cells fall within the same three columns.
The logic is identical: three rows each need the digit once, and the only places available are inside those three columns. So the three columns are fully spoken for, and the digit can be erased from every other cell in them.
The cells do not have to form a neat 3×3 block — a row contributing only two of the three columns is fine. That is what makes a Swordfish hard to see, and why it is worth checking for only after the pair techniques and X-Wing have run dry.