What is a Callback?
A callback is an invitation for a second audition. In most cases, it takes multiple times for an actor to audition before they get the part. Look at it as a singing competition - with each round, the pool of contestants gets smaller and smaller, and in the end only one gets the prize. Same with acting and modeling auditions.
One thing to keep in mind is that callback dates can be pre-determined - so checking your availability before each audition is crucial. It’s also possible for the callback to be canceled, usually because the casting crew have found their talent and are in no need to look at any more contenders.