When three digits in a region can go only in the same three cells, those cells hold nothing else.
Where to lookA pointer first: in row 4, 3 digits can only go in the same 3 cells.Hidden tripleIn row 4, 1, 7, and 8 can only go in column 4, column 5, and column 6, which fills those cells. So they can't be 2, 6, or 9.
Pick three digits in a row, column or box and see which cells each can still go in. If together they are limited to the same three cells, remove every other candidate from those cells.
This example, step by step
Where to lookA pointer first: in row 4, 3 digits can only go in the same 3 cells.
Hidden tripleIn row 4, 1, 7, and 8 can only go in column 4, column 5, and column 6, which fills those cells. So they can't be 2, 6, or 9.
In hints
The hint first gives a direction, a region. Blue ground marks the region, blue marks the three digits kept in the three cells, and orange strikes remove the other candidates.