A casting is a model audition; most likely a modeling agent will send you to it. Usually they are held in photo or casting studios, or sometimes the offices of the brand or magazine. For the casting you will visit a casting director, editor of the magazine, or the art director for a certain job. Let's say the casting involves a hair product ad campaign, and then you should show up with your hair basic, like how it normally is, unless instructed by the person sending you to the casting. Once at the casting you will give the casting director your photo, and you might have your photo taken as well while we are there, so be prepared to smile, or do whatever the vibe of the product is.
You might be told to show a front facial shot, a side profile view, and to smile, or not smile- depending on the vibe of the product. And what the casting director is looking for.
Sometimes the casting director is with the company/product, brand that is looking for model for their ad campaign, etc, sometimes the casting director is hired from the company/product and brand and the casting director takes pics and photos of the models at the casting and then gives them to the brand, company and product's marketing people and they pick a model. It all depends.
You should bring a photo of yourself with you that fits the vibe of the casting. Or your compcard or headshot. Put your agency phone number on it, or if you found this casting yourself put your phone number on it. (always be careful when marketing yourself as a model and trying to find work, there are alot of scams out there. Do not show up for a casting from a post on the internet without looking into it, and asking questions first. It is not worth it to fall prey to a scam or jerk!)