The method Naked Single is the simplest method we all use when we solve Sudoku. We simply observe that
a given cell has all but one 1-9 numbers as neighbours (either in row, column or square). Therefore, there
is only one possibility for the given cell.
Consider the green cell I4 in the following example. It “sees” 1, 2, 6 and 7 in the same row, 5, 4 and 8 in the same column and 1 and 3 in the same square. It can be neither of these values, so the only possibility left for this cell is 9.
Let us look at another example. The cell B4 has neighbours 5, 4, 2 (row), 7, 8, 3, 1 (column) and 7, 9, 4 (square). The only possible value left for B4 is 6.
When you have the candidates option turned on, a Naked Single is simply a cell with only one possible value in it.