A character arc is a fancy way of explaining how a character changes throughout the course of a story. The arc can be physical or emotional, and it doesn’t have to be major, but you want your main characters, especially your protagonist, to experience some sort of change along the journey that is your novel. If your readers get to the end of your book and think “She didn’t learn anything! He’s still so selfish! No one matured at all!” you probably didn’t tell a very interesting story. You want your readers to think the opposite. “Wow! She finally grew up! He learned that hard work does pay! They got what they deserved!” Character arcs satisfy readers, and satisfied readers come back for future books – and tell their friends.