This is a process of elimination that allows candidates to be ruled out rather than directly completing a cell. It applies to both rows and columns but is simpler to explain by discussing one of those at a time.
Within a given subgrid, if all occurrences of a candidate digit are on the same row, then that candidate cannot be used elsewhere on the row (i.e. outside of the subgrid) so those can be eliminated. The same is true for columns. This helps to progress towards the solution, ideally revealing a single or uncovering other candidates to eliminate.
N.B. You should only use this technique once all of the cells on the row (or column) have been filled with candidates. If there are gaps then the candidate digit in question might appear in those gaps which can lead to errors and the puzzle won't be solvable!
In the example, candidate 2 only appears on the third row within the top-left grid. This "points" to the digit 2 not being allowed on the same row outside the grid, which means we can eliminate the candidate 2 in the last cell of the third row.