Where a candidate appears exactly twice in two columns, in the same rows, that candidate can be safely eliminated from other cells in those rows.
Study the image carefully. The bright red-bordered cells form an X-wing. There are 2 cells in the same row of 2 columns that each contain a candidate 3.
There must be a 3 in each of these columns and it must be in one of the two cells. That means that the 3 in the green-bordered cells can be eliminated.
The strategy also works where the candidates appear twice in two rows, in the same columns. That would be a rotation of what you see in the image.