This is u solving technique relies on keeping notes handy. The idea is straightforward: you just have to write the right candidate numbers in all the squares and then see where there are squares with only one candidate number. Let's look at the 3x3 block as an example.
We've already populated the Sudoku grid with the values we wanted (some cells have two values, some have three or more). The goal is to find a cell in the grid where we've only written one value. In this example, we have the lower left block, where the cell contains a candidate number—3. We enter the number and move on to the next step of solving the Sudoku.
Sometimes this Sudoku method is also called "Naked Singles." It's a good idea to use this method for medium Sudoku, as well as for more difficult levels.