A Naked Pair occurs when two cells in the same unit (row, column, or box) have exactly the same two candidates and no others. Since those two values must go in those two cells, they can be eliminated from all other cells in that unit.
Look for pairs of cells within a unit that share the exact same two candidates. This is most useful after you have exhausted all singles.
If two cells in a unit can only contain values A and B, then one cell gets A and the other gets B. No other cell in that unit can contain A or B.
Let's find a Naked Pair. Look at row 1. Several cells are empty. We need to examine their candidates.