A Naked Pair is two cells that contain exactly the same two candidates and are part of the same group. Any other cells in the same group cannot contain the two candidates as those candidates can only be in the Naked Pair cells.
If one of the Naked Pair cells takes a given value, then the other cell in the Naked Pair MUST take the other value, therefore no other cells can have these two candidates in the group containing the Naked Pair.
7 and 9 form the naked pair in Box 1.
That means that candidates 7 and 9 can be removed from cell [2,2].
1 and 7 form the naked pair in Row 8.
That means that candidates 1 and 7 can be removed from cells [8,2],[8,3],[8,5],[8,6].
6 and 9 form the naked pair in Column 1.
That means that candidates 6 and 9 can be removed from cells [3,1],[7,1].
If you tap on the following links on an iOS device which has the Sudoku Tutor app installed, it will launch the app and open the practice puzzle. Tap hint once the puzzle is open to see the strategy in action.Sample Puzzle 1 Sample Puzzle 2 Sample Puzzle 3