Pointing Pair

Look at where a digit can still go within a single 3×3 box. If every one of those candidate cells sits in the same row (or the same column), then the digit must end up somewhere in that row within the box.

That means the digit can safely be crossed off as a candidate everywhere else on that row or column, outside the box — the box is 'pointing' its candidates for that digit along a line.

Suppose in a box the digit 4 can only go in two cells, and both sit in the box's middle row. Every other cell in that row outside the box can then have 4 crossed off — the box guarantees a 4 lands in one of those two spots, so no cell elsewhere in the row is allowed to hold one.