A naked single is a cell that can hold only one digit: every other digit is already in its row, its column or its box.
In Sudoku X a cell on a diagonal has a fourth house. The digits on that diagonal rule digits out too, so a cell can be settled by its diagonal when its row, column and box leave two or more.
Worked examples
2 positions from real Sudoku X 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 1. One empty cell here has only one candidate left.
The diagonal settles it
This cell is on a diagonal. Its row, column and box leave more than one digit here; the lit diagonal is what decides it.
Where it fits
EasyNaked Single is an Easy technique: Easy Sudoku X puzzles are the first to need it.
The Classic Naked Single lesson teaches it on a plain 9×9 board. The Sudoku X overview explains the rule and the levels.
Naked Single in the other styles: Jigsaw, Thermo, Windoku, Arrow, Kropki.
Try it on an Easy Sudoku X puzzle. Next Step points it out when it is the move.