When two cells in a house hold only the same two candidates, no other cell in the house can use those two digits.
On a Thermo board this works exactly as in Classic. The examples are from Thermo puzzles on this site. The thermometers often make the pair, by cutting a cell down to two candidates.
Worked examples
A position from a real Thermo puzzle on this site. Step through it: the board lights what each step talks about, and the arrows show what rules what out.
A pair in a row
In row 2, look for two cells that hold the same two candidates between them.
Where it fits
HardNaked Pair is a Hard technique: Hard Thermo puzzles are the first to need it.
The Classic Naked Pair lesson teaches it on a plain 9×9 board. The Thermo overview explains the rule and the levels.
Naked Pair in the other styles: Sudoku X, Jigsaw, Windoku, Arrow, Kropki.
Try it on a Hard Thermo puzzle. Next Step points it out when it is the move.