Pointing
Pointing occurs when every remaining candidate for a digit in a box lies in one row or column. The box must place the digit on that line, so the digit can be removed from the same line outside the box.
What is Pointing?
Pointing — sometimes called box-line reduction — is usually one of the first intersection techniques you learn after singles. Inside one 3×3 box, all candidates for a digit lie on the same row or column. That digit must occupy the intersection, so the same candidate can be removed from the rest of the line outside the box.
How to spot it
Flip through the boxes one at a time and look for a digit whose candidates all sit on one row or one column. With pencil marks on, this jumps out — the marks for that digit form a little line inside the box instead of scattering. Once you see it, follow that row or column outside the box and erase the digit wherever it appears. Pointing shows up a lot in the first half of a solve, when the grid is still crowded with candidates and one clean elimination can open things up.
The logic
Every box needs each digit 1–9 exactly once. If a digit's only candidates in a box are all on one row, one of those cells has to be the digit — there's nowhere else in the box it can land. But a row only holds each digit once, so once the box uses up that digit on the row, no other cell on the same row can have it. That's why every candidate outside the box on that row can be crossed off.