Hidden Pairs
Sudoku Solving Technique
The Hidden Pairs technique is the inverse of Naked Pairs. It can be harder to spot because the pair is "hidden" among other candidate numbers in the cells.
How it Works
You are looking for a situation where two candidate numbers appear in exactly two cells within a specific group (row, column, or block), and nowhere else in that group. Even if those two cells have other pencil marks in them, the fact that our two target numbers can ONLY go in those two cells means they must be the answer.
Therefore, you can safely delete all the "extra" candidate numbers from those two cells, effectively turning the Hidden Pair into a Naked Pair.