I got it ! Row 2 column 1 can't be a 4 because 4 has to go in column 3. Row 2 column 1 can't be a 3 because 3 has to go in row 2 column 4 or 5. That is the thought process right?!
Since the three open cells (row 2, columns 3, 4, 5) can ONLY contain the three digits (2, 3, 4), those three cells form a triplet and eliminate 2, 3, 4 from all other open cells in the same row. Another way to think about it is that I only marked down 2, 3, 4 in those three cells, so the missing digits of row 2 (1, 5, 6, 8) can only go into the other four open cells, eliminating 2, 3, 4 from R2C1. If you can't process or still don't understand the logic behind, you may try substituting 3 or 4 into R2C1 and see what happens, the result: one cell in row 2 would end up being empty & one of the digits cannot be placed. You may view the concept from different angles, but nevertheless, hope the explanation helps :)
