Like the "Obvious Singles" technique, "Obvious pairs" is based on the correct placement of Notes. The point is that you should find 2 cells with the same pairs of Notes within 3x3 block. This means that these pairs of Notes cannot be used in other cells within this 3x3 block. So they can be removed from your Notes. It will be easier to understand this strategy if you look at the example.
Let's look at this block. We see empty cells filled with notes of possible numbers. Among them, there are two cells that contain 7 or 9.
This means that one of these cells necessarily contains 7 and the other one contains 9. This also means that we can't have 7 and 9 in other cells of this block.
Hence, we remove them from other cells' notes. Next we can apply the "Obvious singles" rule we learned in the previous lesson. We'll write 6 in the cell with a single note of 6 and and 4 in another one.
That is how "Obvious Pairs" technique can be used while solving Sudoku. Once you have learned it, you can proceed with the following Sudoku strategies.