naked pairs. They are harder to see because of the presence of other candidates in the two cells involved. Unlike naked pairs, the "pair of candidates" exist only in two cells, plus there are other candidates besides the pair in these two cells. Since these two candidates do not exist anywhere else in the row, column or box, the two cells involved must have these two candidates as the solution - hence we can eliminate the other candidates in these two cells.
The difference between the naked pair and hidden pair is illustrated in the two row examples below. Look at the cells with candidates marked in red. In the naked pair, the candidate pair [1,7] are the only candidates in the two cells but exist elsewhere on the row (we eliminate the candidates 1 and 7 elsewhere in the row). In the hidden pair, the candidates in the pair [1,7] are only in two cells but these cells have other candidates (we eliminate the extra candidates 3, 4 and 7 in these two cells).
Hidden Pairs can form in rows, columns or boxes. Formation in a row has already been illustrated above. Formation in a column and a box are illustrated below.
In the first example below, the column contains the hidden pair [5,6] in the two cells marked in red. We can eliminate 1, 3, 4 and 9 from these two cells. In the second example below, the box contains the hidden pair [4,6]. We can eliminate 3,5,8 and 9 from these two cells.