If only two candidates occur in such a pair of cells, these two candidate numbers must go into these two cells - we cannot say which cell contains which, but we can say that these candidates cannot exist anywhere elsewhere on that row, column or box.
In the first example below, row 9 contains the naked pair [5,9] (marked in red). Since these two cells can have only the two candidates 5 and 9, if '5' is in one cell, then '9' is in the other, and vice versa. These two candidates have to be in these two cells and cannot be elsewhere on that row. Thus we can eliminate 5 and 9 elsewhere in that row.
In the second example below, column 8 contains the naked pair [5,8]. We can eliminate 5 and 8 as candidates elsewhere in that column. (Note: Only the area of interest is shown in the puzzle.)
Note that a given pair of cells can form a naked pair on both a row and a box, or both a column and a box.
In the first example below, the pair [3,4] are in both row 3 and the top middle box. We can eliminate 3 and 4 elsewhere in row 3 and in the top middle box. In the second example, the pair [1,9] are in both column 8 and the middle right box. We can eliminate 1 and 9 elsewhere in column 8 and in the middle right box.
Note that, technically speaking, the following pattern [3,4] is also a naked pair (even though we would normally solve the single candidate first).