Definition of showdown
Showdown is the last step in a hand of poker's. All hands of poker's have no bust, only some. Showdown is when the remaining players in a hand must show their cards to determine the winning hand. Showdown is after the river and the player who wants to put his hand on the jackpot will have to show his cards. In some situations, if you don't have the best hand and don't want to show your cards so as not to give information to your opponents, you can simply discard your cards (muck). You obviously lose the jackpot.
Some more information about showdown
The player who will win the showdown jackpot must show his cards to all players, who will be able to take note of how he played them. Many aggressive and bluffing players will try to avoid going to the showdown to win as many uncontested pots as possible, i.e. pots that will be won before showdown. Players called "calling station", who are very quick to call bets, will be players who want to see a lot of showdown to make sure they are not bluffed. They will also be called Shérifs, because they will always want to make sure they are not bluffed.