A naked single is a cell that can hold only one digit. In Classic its row, column and box rule the others out.
In Thermo a cell on a thermometer is also limited by its place on it, so a cell can be settled even when its row, column and box leave two or more digits.
Worked examples
2 positions from real Thermo 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 R2C4, on a thermometer. Only one candidate is left here.
A single on a thermometer
This cell is on the lit thermometer. Its row, column and box leave more than one digit; its place on the thermometer rules out the rest.
Where it fits
EasyNaked Single is an Easy technique: Easy Thermo puzzles are the first to need it.
The Classic Naked Single lesson teaches it on a plain 9×9 board. The Thermo overview explains the rule and the levels.
Naked Single in the other styles: Sudoku X, Jigsaw, Windoku, Arrow, Kropki.
Try it on an Easy Thermo puzzle. Next Step points it out when it is the move.