A naked single is a cell that can hold only one digit. In Classic its row, column and box rule the others out.
In Arrow a cell on an arrow is also limited by the sum: a circle is at least what its arrow adds up to, and a cell on the arrow at most what the circle leaves. So a cell can be settled even when its row, column and box leave two or more digits.
Worked examples
2 positions from real Arrow puzzles on this site, the familiar case first. Step through each one: the board lights what each step talks about, and the red lines show what rules what out.
Settled by its row, column and box
Look at row 1. One empty cell here has only one candidate left.
A single on an arrow
This cell is on the lit arrow. Its row, column and box leave more than one digit; the sum the arrow must make rules out the rest.
Where it fits
EasyNaked Single is an Easy technique: Easy Arrow puzzles are the first to need it.
The Classic Naked Single lesson teaches it on a plain 9×9 board. The Arrow overview explains the rule and the levels.
Naked Single in the other styles: Sudoku X, Jigsaw, Thermo, Windoku, Kropki.
Try it on an Easy Arrow puzzle. Next Step points it out when it is the move.