If more than one player is still involved in the pot after all of the rounds of betting have been completed, the hand goes to a showdown. This is when the remaining players show their hands to determine who has the best and who wins the pot.
While all remaining players have the option to show their cards, in many cases players with losing hands will choose to simply toss their cards away without revealing them (known as "mucking") if they see that a stronger hand has already been shown.
If you are playing online, you will be offered the option of mucking a beaten hand or showing it to the table.
The video covers the rules regarding which player needs to show their cards first. Most players prefer to never show their cards unless they have to.