This technique of solving Sudoku requires the active use of notes. The application is simple: you have to write suitable candidate numbers in all squares, and then see where there are squares with the only candidate number. Let's take the 3*3 block as an example.
We have already filled the Sudoku grid with the desired values (some cells have 2 values, some have 3 or more values). The task is to find a cell in the grid where we have written only 1 value. In this example, we have the lower left block, where the cell contains a candidate number - 3. We enter the number and continue solving the sudoku.
Sometimes this Sudoku method is also called "Naked Singles". It is better to use this method for medium sudoku, as well as for more difficult levels.