"Naked Single" Sudoku solving technique explained:
A Naked Single means that in a specific cell only one number remains possible.
This applies after eliminating all the other candidates by using different solving techniques. In other words, a naked single is the last remaining option and cannot hide there for it’s a “Naked Single”.
Take a look at the example:
look at the block marked in pink, the cell marked with a blue square has only option - 6