What is a Naked Pair?

A Naked Pair occurs when two cells within the same group (a row, a column, or a 3x3 box) contain exactly the same two candidates, and no other candidates.

Why are they so powerful?

If cell A can only be 2 or 5, and cell B (in the same row) can also only be 2 or 5... then we know for an absolute mathematical certainty that the 2 and the 5 MUST go into those two cells. We might not know which goes where yet, but they will exclusively occupy those two spots.

Because they occupy those two spots, you can safely eliminate the numbers 2 and 5 from every other cell in that row! This simple elimination often cracks open the rest of the puzzle.