"Last remaining cell" technique.

"Last remaining cell" is another basic Sudoku strategy. It's based on the fact that numbers should not be repeated within 3×3 block, vertical column and horizontal row.

Let's take a look at an example with the 3x3 block. There always must be number 8 - in each block, column and row. There's already 8 in the column and in the row. As we already know, we can't repeat numbers. So we can't place 8 there again. It means that there's only one cell remaining inside the block and we should put number 8 into it.

The same technique applies to the Rows and Columns. 

That is how "Last remaining cell" technique can be used while solving Sudoku. Once you have learned it, you can proceed with the following Sudoku strategies.