This strategy is the inverse of naked triples and can be used to clarify remaining candidates in the grid, leading to other solving opportunities. It applies to rows, columns and subgrids but is easier to explain by discussing one of those at a time.
You're looking for a triplet of candidate numbers used only in three different cells within the same subgrid. They are harder to spot than naked triples as other candidates can also be present. As the trio of cells must contain all occurences of the tripled digits (but we don't know which order just yet), we can eliminate any other candidates in the tripled cells.
N.B. You should only use this strategy once all cells in the subgrid (or row or column) have been filled with candidates.
In the example, the numbers 3, 6 and 7 form a hidden triple as they are only used in three cells of the subgrid. We can then eliminate any other candidates in those three cells turning them into a naked triple.