Anything pertaining to developing the details associated with characters in your writing.
Character development pertains to anything that is involved with identifying and detailing specific information about your story's characters. What kind of relationships does your character have with other characters in the story? What motivates your character to do something relevant in the story? What are the character's strengths or weaknesses? Any of these questions can be used to help you develop a better picture of who the character is and how he will affect your story.