This technique is used to eliminate pencil numbers. Use pencil numbers to mark the possible numbers for each cell.

A naked pair is where, within a row, column or jigsaw piece, there are two squares with only the same two pencil numbers remaining.

Each of these cells must be filled with one of these numbers, though we don't know which one is which yet. As a result, they cannot appear anywhere else within the same area.

For example, the highlighted cells are the only place 1 or 2 can appear in this column. 

As a result, the 2 can be eliminated from the other cells in this column because it must appear within one of the highlighted cells.