The character arc is the journey that the lead character follows in a story and changes, hopefully for the better by the end of the novel. This article is a good one for detailing what a character arc is and how to develop one.
A number of writing instructors refer to the character arc, or the path a protagonist takes from the beginning of the story to the end, as a tool for the novelist. In many respects it is an artificial construct best recognized after the fact. Except, what happens to the main character really is the story.
Some novels or visual stories, to be sure, have heroes who change little. Captain Kirk of the USS Enterprise in the original Star Trek TV show exhibited little change week after week. The Lone Ranger, first as radio drama, then as a 50s black and white TV favorite, centered on a similar unchanging hero. Read more.