A naked single is a cell that can hold only one digit. In Classic its row, column and box rule the others out.
In Kropki the cells beside it count too: each placed neighbour rules digits out, across a dot or across an edge with no dot. So a cell can be settled when its row, column and box still leave two or more digits.
Worked examples
2 positions from real Kropki puzzles on this site, the familiar case first. Step through each one: the board lights what each step talks about, and the arrows show what rules what out.
Settled by its row, column and box
Look at row 3. One empty cell here has only one candidate left.
The dots settle it
Its row, column and box leave more than one digit here. Look at the cells beside it, and the dots or missing dots between them.
Where it fits
EasyNaked Single is an Easy technique: Easy Kropki puzzles are the first to need it.
The Classic Naked Single lesson teaches it on a plain 9×9 board. The Kropki overview explains the rule and the levels.
Naked Single in the other styles: Sudoku X, Jigsaw, Thermo, Windoku, Arrow.
Try it on an Easy Kropki puzzle. Next Step points it out when it is the move.