The "Last possible number" is a simple strategy suitable for beginners. It is based on finding the missing number. To find the missing number, you should look at the 3x3 block of interest and see which numbers are already present in its connected row and column.
Let's look at an example.
Pay attention to the highlighted cell. Look at the numbers in its block, row, and column. We can see that this row, column, and block already contain the numbers 1, 2, 3, 4, 6, 7, 8, and 9.
The only missing number is 5. Since numbers cannot be repeated, the only number that can be placed in this cell must be 5.
This is the principle of the "Last possible number" solving method. Once you master it, you will start solving Sudoku puzzles more easily and quickly!