The technique is based on the concept of "obvious pairs." To solve the Sudoku, you need to find three squares with the same numbers.
The method might seem a bit complex at first, but it's actually pretty straightforward in practice. Let's take a look at the following example. The numbers missing from the line are 1, 2, 6, 7 and 9. The 'last possible number' method basically means that in the middle 3x3 block there'll be a number 1, and in the 3x3 right block there can only be a number 7. So, we can just cross these numbers out from the bottom left row in the left block.
We've already ruled out numbers 1 and 7 as candidates. In the left block, the candidate numbers are still 2, 6 and 9. These are our "obvious triples".
Let's take a look at the left 3x3 block now. The bottom three cells can only be filled with two, six, or nine, and no other digits are allowed (that's how Sudoku rules work). So, we can get rid of these numbers from the other cells. We remove the nines (there aren't any twos or sixes left in this block). In the middle left cell, we have an obvious singleton, which we'll write as 1.
Next, we take the 1 out of all the notes and add the numbers 3 and 9.
The "Obvious triples" method helps you to focus your search and avoid mistakes caused by duplicate values.
Once you've got the hang of this method and can solve simple Sudoku puzzles, you can move on to more challenging ones and try out other strategies.