Here is a quote I found online to possibly help you understand: "character development describes the change an individual undergoes through the course of a story as a result of the conflict or conflicts that person encounters. Think of the character Scrooge from A Christmas Carol, who begins as a crotchety, tight-fisted, greedy old man, but over the course of the story, various "conflicts" force him to change his ways so that, by the end, he becomes a better man, generous and kind."

Character development means how the character grows or changes in the story. So, for Squid Game, the character development for Gi-Hun is that at first he is very selfish (he isn't taking care of his family, for example). By the end of the drama, Gi-Hun tries to help the families of his friends that died by giving money to them, and he wants to stop the Squid Game from happening again. Before, he was selfish; now, he is more caring. So that is one example of character development.

By the way i believe it means the changes in moods, personalities of actress.