What does it mean when a casting director calls you back? It means you "showed up" for the audition and gave the audience a taste of what you can do, and how you would act in that character's shoes.
How do casting directors decide who to pick? You can try forever to figure this out, but here's my summary of the factors that count.
What's Your Flavor?
Understand that directors bring back the actors who know themselves, and who feel confident with how they interpret the material within themselves.
Everyone actor invited to a call back is a strong candidate, and each actor brings his/her own characteristics to the read. Go ahead and be different. Think of yourself as a ice cream flavor. Your flavor is like know one else's. Understanding what flavor you are is the key to success. While the call back is in session, the producers, director, and casting people are watching good actors express their uniqueness and your job is to show them that your flavor best represents the character.
Who gets chosen for the part may be out of your control, but you can control what you present. Even though your flavor might not be picked this time for this piece, coming prepared may mean that they keep you in mind for next time.
What is your unique flavor? Bring to the table it could very well get you booked.