People who believe that selection, or choosing the best, is very important, especially in things like art or culture.
Selectionists think it's really important to choose the best things, like the best art or the best music.
Examples:
The selectionists believe that only the best art should be shown in the museum.
The selectionists argue that only the best books should be in the library.
The selectionists think that only the best music should be played on the radio.
The selectionists want only the best food in the restaurant.
The selectionists insist that only the best players should be on the team.
The selectionists say that only the best words should be used in the story.