Naked pairs are a technique used in solving Sudoku. If you have two cells in a group with the same two candidates and no others, you can eliminate those two candidates from the rest of the group. Naked pairs are a special case of the more difficult (both to identify and to use) naked subset pattern.
The following diagram is a box from a partially solved Sudoku board. Each cell in this diagram contains the candidates for the cells in one box of the Sudoku board.
*-------------------------*
| 19 19 34 |
| 1579 1789 3478 |
| 567 678 2 |
*-------------------------*
Notice the two hardlinked cells with candidates one and nine. Clearly, one of them is going to be a one, the other is going to be a nine. Therefore, we can remove candidates one and nine from the rest of the box, because, while we don't know the exact details, we know the one and nine are going into the top-left and top-center cells. If we had more of the board, we could also eliminate one and nine from the rest of that row as well. After removing the impossible ones and nines, we end up with the following, which may fall prey to our friend and ally, the hidden single (perhaps now there's only one nine in the column containing the middle of this box). Affected cells are hardlinked.
*-------------------------*
| 19 19 34 |
| 57 78 3478 |
| 567 678 2 |
*-------------------------*
Simple Sudoku makes this pattern easy to find. Ctrl+Y turns on filtering for all cells with exactly two candidates. From there you can just look at each pair and try to find buddies with the same two candidates.
And now some boards that require naked pairs (and easier techniques).
*-----------* *-----------* *-----------*
|..1|6.8|.9.| |2..|...|..5| |..3|...|.6.|
|..8|..7|6..| |47.|..6|..8| |...|5..|.19|
|5..|2..|..3| |..6|.7.|...| |24.|6..|8..|
|---+---+---| |---+---+---| |---+---+---|
|..3|...|418| |..8|.5.|..4| |.9.|4..|...|
|...|...|...| |..4|8.1|5..| |..8|.9.|4..|
|865|...|3..| |5..|.2.|1..| |...|..1|.8.|
|---+---+---| |---+---+---| |---+---+---|
|3..|..5|..4| |...|.1.|3..| |..9|..5|.21|
|..7|3..|5..| |3..|6..|.17| |32.|..8|...|
|.5.|7.1|9..| |8..|...|..9| |.6.|...|7..|
*-----------* *-----------* *-----------*