"Last free cell" is a basic Sudoku solving method. This method is very simple and is based on the fact that every 3x3 block, every row, and every column on a Sudoku board must contain the numbers 1 through 9, and each number can only be used once in each of them.
Therefore, if we see a 3x3 block, a row, or a column with only one empty cell remaining, we must determine which number from 1 to 9 is missing and fill it into that empty cell.
You can see what this looks like in the example below.
This is the main basic rule. After you have learned it, you can move on to the next Sudoku strategies below.