Box/Line Reduction is the reverse of Pointing Pairs. When a candidate in a row or column is confined to a single box, it can be eliminated from all other cells in that box (outside the row or column).
For each row or column, check each candidate. If a candidate only appears in cells that belong to the same box, it can be eliminated from other cells in that box.
The row or column must contain that digit, and it can only place it within one box. Therefore, that digit is locked to the intersection, and other cells in the box cannot hold it.
Let's look for a Box/Line Reduction. Examine row 1 and see where digit 5 can go.