"Naked Single" Sudoku solving technique explained:

sudoku showing the naked single technique

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

sudoku demonstrating naked single solution

It’s also known to some by the names "Sole Candidate" or "Forced Digit".