An audition is where an actor gives a short performance so the casting director can determine whether he is right for the part.
If you didn't receive sides (an excerpt from the script), you will be expected to perform a monologue. Do the same one you originally auditioned with and wear the same outfit. Bring another copy of your head shot and resume.
After you've performed your monologue or sides, the casting director might ask you to do it again in a different way or even perform a scene with another actor. Be open to all suggestions.
Don't suck up to the casting director or try to socialize. Be professional, courteous and friendly, but get in and out of the audition as efficiently as possible.
Keep a log of which auditions you went to, who the casting director was, what monologue you performed, what you wore, what feedback you received and any other pertinent details so you don't forget. Send a thank you note to the casting director, remain hopeful and go on to the next audition.