What is Naked Pair? How to Solve Sudoku

What it is

If two cells in the same row (column or box) can each only be 3 or 8, then 3 and 8 are used up by those two cells — even if you don't know which is which yet.

So no other cell in that row can be 3 or 8.

How to spot it

When you find a cell with only two candidates, look in its row, column, and box for another cell with exactly the same two.

Example

Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the blue digit is the answer.

In box 1, R1C1 and R1C3 can only hold 2 and 9. That's 2 cells for 2 digits, so 2 and 9 are used up here (even if we don't know which goes where yet). So the other cells of box 1 (R1C2 and R2C2) can't be 2 and 9.