Last possible number is a simple strategy that is suitable for beginners. It is based on finding the missing number. To find the missing number you should take a look at the numbers that are already exist in the 3x3 block you are interested in, and in the rows and columns connected with it.
Let's take a look at an example.
Pay attention to the highlighted cell. Look at the numbers in its block, row and column. We can see that numbers 1,2,3,4,6,7,8,9 are already used in this row, column and block.
The only missing number is 5. Considering that numbers should not be repeated, the only number that must be put in this cell is 5.
That is how "Last possible number" technique works. Once you have mastered it, you will begin to solve Sudoku easier and faster!