A Pointing Pair (or Triple) occurs when a candidate within a box is restricted to a single row or column. Since the value must appear in that box, and it can only be in cells along that row or column, it can be eliminated from other cells in the same row or column outside the box.
For each box, check each candidate. If a candidate only appears in cells that share a row (or column), it is 'pointing' along that line, and you can eliminate it from the rest of that line.
The box must contain that digit, and it can only go in cells along one line. Therefore, that line's portion inside the box is claimed, and the digit cannot appear elsewhere on the same line.
Let's look for a Pointing Pair. Examine box 1 (top-left 3x3). We want to find a candidate that is restricted to a single row or column within this box.