Pointing is the first technique on this site that doesn’t place a digit — it only removes candidates. That’s a shift worth noticing: most of solving past the easy stage is elimination, not placement, and pointing is the simplest version of that move.
What “pointing” means
Look inside a single 3x3 box. If a digit’s remaining candidates there all sit in the same row (or the same column), that digit is “locked” to that row within the box — it has to go in that box, and it has to go in that row. Which means it can’t go anywhere else in that row, outside the box.
That’s the whole technique: the digit is confined to one line inside the box, so it “points” out of the box and clears itself from the rest of that line.
Worked example
In box 1, 6 only appears in column 1, so it can be removed from the rest of that column.
Notice the cells that lose the candidate aren’t in the box at all — they’re elsewhere in the same row (or column), outside it. That’s the part people miss the first few times: the elimination happens away from where the pattern was spotted.
How to find one quickly
Pick a box with three or more digits still unplaced. For each of those digits, mark (mentally or on paper) every cell in the box it could still go in. If those cells all line up in one row or column, check the rest of that row or column outside the box — any cell there that still lists the digit as a candidate can have it crossed out.
Related techniques
Claiming is the mirror image of this: start from a row or column instead of a box, and eliminate inside the box instead of outside it. Once pointing and claiming both stop helping, naked pairs is usually the next thing to check.
Puzzles at Medium are the first level built around needing this.