A Jellyfish is the 4-row extension of X-Wing and Swordfish. A candidate appears in at most four positions in each of four rows, and all positions fall within exactly four columns. The candidate can be eliminated from all other cells in those four columns.
After X-Wings and Swordfish, look for four rows where a candidate is restricted to the same four columns. Each row may have the candidate in two, three, or four of those columns.
Each of the four rows must place the candidate once, and all placements fall in four columns. Since four placements fill four columns, no other cell in those columns can hold the candidate.
Let's find a Jellyfish — a 4-row fish pattern. We focus on digit 5. We look for four rows where 5 appears in positions spanning exactly four columns.