When all of a digit's places in one region lie inside another region, the rest of that other region cannot hold it.
Where to lookA pointer first: look at row 2. Where can 2 still go?Locked candidatesIn row 2, 2 can only go in column 5 or column 6, and those two cells lie in the top box. So no other cell in the top box can be 2.
Look at a box where a digit can go only in cells of one row. The digit must then be in that box on that row, so remove it from the row's cells outside the box. The same holds with columns, and with a row whose places all lie in one box.
This example, step by step
Where to lookA pointer first: look at row 2. Where can 2 still go?
Locked candidatesIn row 2, 2 can only go in column 5 or column 6, and those two cells lie in the top box. So no other cell in the top box can be 2.
In hints
The hint first gives a direction, a box or a row. Then blue ground marks the region, blue marks where the digit can still go, and orange strikes remove the candidates in the other region.