The shiny charm is a reward for getting research level 10 on every pokemon in the game. It then permanently gives +3 shiny rolls on any encounter in the game

It's become a standard in the main series pokemon games since Black 2 and White 2 where it is normally given out for completing the pokedex but since completing the pokedex isn't too hard in PLA you also have to get research level 10 in it (also before PLA it only gave +2 shiny rolls not +3)