A Hidden Triple occurs when three values appear as candidates in exactly three cells of a unit (and nowhere else in that unit). Those three cells must contain those three values, so all other candidates can be removed from those cells.
After checking for pairs, look for three digits that are each confined to the same three cells in a unit. This is rarer than Hidden Pairs but follows the same logic.
If three values can only go in three cells of a unit, those cells are fully occupied by those values. Any additional candidates in those cells are impossible.
Let's look for a Hidden Triple in box 5 (the center box). Three cells are given (8, 3, 7, 9) and five cells are empty.