This is an advanced strategy that can be applied to remove some uses of a candidate in the grid, leading to other solving opportunities. It applies to rows or columns but not to subgrids.
You're looking for a single candidate digit that appears exactly twice in two columns and at least twice in two intersecting rows so that an imaginary rectangle shape is formed with the digit at each corner. The digit can only appear once per row and once per column, in two of the diagonally opposite corners of the rectangle so we can eliminate any other uses in the intersecting rows. This is known as an X-wing in the columns.
Alternatively we can identify a digit that appears in exactly two rows and at least two intersecting colums, again forming a rectangle. We can then eliminate the candidate from other locations in the intersecting columns. This forms an X-wing in the rows and is shown below.
N.B. You should only use this strategy once all cells in the relevant columns and rows have been filled with candidates.
In the example, the digit 2 appears exactly twice on the second and fith rows forming a rectangle with the intersecting second and sixth columns.
We can therefore eliminate any other uses of 2 in the second and sixth columns, revealing a couple of naked singles: