The method Hidden Single is similar to a Naked Singles in the sense that a given cell will have only one
possible value. The elimination is done differently. A collection of cells with the same number will force
(pin down) the number in a given cell.
The following example illustrates a Hidden Single in B2 with respect to row B. Consider row B and the cells with value 9 in blue. There cannot be a 9 in cells B1, B4, B6 and B8, because the blue cells contain a 9 and are aligned in columns 1, 4, 6 and 9. Hence, the only possible place for a 9 in row B is in B2. Note that if we just focus on the possible values of a given cell (as in Naked Singles), then B2 can be either 6 or 9 and both will be possible. The Hidden Single elimination considers the positions of all (or some) cells with value 9 and not just the possibilities for a given cell.
In the previous example we looked at a Hidden Single in a row. In fact we can do this for columns and squares in the same way. Here is an example where we consider the top-left square. The number 9 cannot be placed in A2, B2 or C2 because of the blue G2 cell with value 9. Hence the only possibility for C1 is 9.
If you have the candidates option turned on, a Hidden Single can easily be found as a cell where a given candidate appears only once in its row, column or square. In this example, candidate 8 in cell C5 appears only once in column 5. Therefore, it is a Hidden Single and C5 has value 8.