A naked single is a cell that can hold only one digit: every other digit is already in one of its houses.
A cell inside a window has a fourth house and sees 23 or 24 other cells instead of 20, so the window often supplies the last digit that rules the others out.
Worked examples
2 positions from real Windoku 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 2. One empty cell here has only one candidate left.
The window settles it
This cell is inside the lit window. Its row, column and box leave more than one digit; the window rules out the rest.
Where it fits
EasyNaked Single is an Easy technique: Easy Windoku puzzles are the first to need it.
The Classic Naked Single lesson teaches it on a plain 9×9 board. The Windoku overview explains the rule and the levels.
Naked Single in the other styles: Sudoku X, Jigsaw, Thermo, Arrow, Kropki.
Try it on an Easy Windoku puzzle. Next Step points it out when it is the move.