Where 3 rows or 3 columns contain up to 3 of the same candidate in the same relative position, that candidate can be eliminated from other cells in the group.
It's hard to write a snappy definition of the Swordfish technique but it isn't too hard to understand when you see how it can be used in a puzzle.
The red-bordered cells form a Swordfish pattern.
They each contain a candidate 4.
Each of these rows must contain a 4 and you can safely eliminate the 4 from any other cell that would prevent us placing a 4 in our Swordfish pattern.
The 4 can be eliminated from all the green-bordered cells.