A Hidden Single occurs when a candidate appears in only one cell within a row, column, or box. Even if that cell has multiple candidates, the value must go there because no other cell in that unit can hold it.
After checking for Naked Singles, scan each unit (row, column, box) and count how many cells can hold each digit. If a digit can only go in one cell of a unit, that is a Hidden Single.
Every unit must contain each digit exactly once. If a digit has only one possible location in a unit, it must go there regardless of what other candidates that cell has.
Let's find a Hidden Single. Look at row 4 (cells R4C1 through R4C9). Several cells are empty with multiple candidates each.