For example in Texas hold'em, if the board is 5♠ 6♠ A♣ 9♠ 5♥, a player holding 7♠ 8♠ has the nuts (an 9-high straight flush in spades), and cannot lose. Sometimes it is useful to know that your hand is the second or third best possible. On this same board, the hand 5♣ 5♦ would be the second-nut hand, four fives; and the third-nut hand would be any pair of the remaining three aces, making a full house A-A-A-5-5.
In High-low split games one often speaks of "nut low" and "nut high" hands separately. With an Omaha board identical to the one above, any hand with 2-3 makes the nut low 6-5-3-2-A, while 2-4 is the second-nut low (the nut high hands remain the same).
Finally, one also hears terms such as "nut flush" or "nut full house" to mean the highest hand possible in that particular category under the circumstances, even though that may not be strictly the nut hand. For example, a pair of aces with the above board could be called the "nut full house", even though there are two higher (but very unlikely) hands possible.