"Last free cell" technique.

"Last free cell" is the basic Sudoku solving technique. It's pretty simple and based on the fact that each 3×3 block, vertical column or horizontal row on Sudoku grid should contain numbers from 1 to 9 and each number can be used only once within 3×3 block, vertical column or horizontal row.

Therefore, if we see that there is only one free cell left in the 3×3 block, vertical column or horizontal row, then we have to define which number from 1 to 9 is missing and enter it in this empty cell. 

You can see how it looks in the examples below.

This is the main basic rule. Once you have learned it, you can proceed with the following Sudoku strategies.