Definition of showdown
The showdown is the culling. It is the last street in a poker hand. If we divide the hand, there is the preflop action, the flop, where 3 community cards are dealt, the turn, where one more community card is dealt, the river, where the last community card is dealt and finally, if applicable, the showdown.
Hands that are won on showdown and without showdown
If you have a tracker like PokerTracker 4 and you analyze your play, you have probably seen that PokerTracker divides your winning hands into two categories, showdown wins and non-showdown wins. If you win the majority of your hands on showdown, you are not very aggressive and are probably a tight player, but if you win the majority of your hands without showdown, you are very aggressive. For the hand not to go to showdown, it took a high level of aggression to get your opponent to fold. Generally, players will do much better on showdown than without showdown.