The Selection is a competition to fight for the heart of the "One". For example, if the "One" was a guy, so many lady competitors (number can change) compete with each other to win his heart. There are activities for the "One" to do with each of the competitors and every couple days a couple ladies will be eliminated as the "One" continues to find the one that stole his heart. After the "One" chooses his lady out of the selected the couple will celebrate with a party that acts as an activity check. At that party the next "One" will be announced. 

It is not required that the couple at the end officially date. I understand that the "One" might not find his true soulmate among the lovely ladies but it is definitely encouraged since usually (from past experiences) it does happen. This is a fun dramatic way to find the love of your life. And sometimes you find it without even trying.