A "Pointing pairs" strategy applies when a candidate within a 3x3 block appears only two times, and both instances fall on the same row or column. This means the solution for that candidate within this block must be in one of those two cells. Therefore, you can remove that candidate from the notes of all other cells in that row or column outside of the block.
To better understand "Pointing pairs", let's look at an example.
Let's look at the top-left block. All the cells that could possibly contain the number 4 are in a single column. Since the number 4 must appear at least once in this block, one of the highlighted cells must contain the 4.
From this, we can safely eliminate the possibility of 4 being in any of the other cells in this column.
Remember that you can use the same technique for blocks, rows, and columns.
This is the "Pointing pairs" solving method. Now you can move on to the next Sudoku strategy below, "Pointing triples."