To ruff a trick that someone has already ruffed, with a higher trump. Although usually it is a good idea to overruff when you get the chance, if you would have to play a very high trump to do so and you have a useful discard to make, it may be better not to do so. Also, if an opponent threatens to overruff you, again it might be better to discard.
South plays in 4♠ after East has bid hearts. The defenders start with three rounds of hearts. If South ruffs the third round of hearts low, West can play the ten (or jack) of spades to overruff. South does better to discard his diamond loser. Then, if East persists with a fourth heart and West ruffs with an honour, dummy can play the queen to overruff.