How a naked pair works
Suppose r4c4 and r4c8 each have exactly the candidates 2 and 9. Since every digit appears once in row 4, those digits must occupy these two cells in some order.
We do not yet know which cell is 2 or 9. However, both candidates can be eliminated from all other cells in row 4.
Avoid common mistakes
Exactly two cells in the unit must contain the same two-candidate set. Three cells with the same pair are not a naked pair. Both cells must belong to the same row, column or box.
This technique often makes a single appear elsewhere rather than placing a digit directly.