When there is an 'open' casting call that means anyone can go and audition, but usually, as in this case, there are separate times for Union and Non-Union actors. This video shows you the Non-Union people interested in being Background Actors (aka 'Extras') for the new television series "Gotham" airing on FOX in Autumn 2014. Most people chosen will be background actors simply walking around in the scene, acting natural while some will have more featured roles or may even get speaking parts.
You never know on a television or movie set. You might show up in the morning as an extra and end up getting promoted to a higher pay grade and role because they need someone who looks, walks, and talks like you for a sudden idea the director has or because some dummy didn't show up.
One key thing to know when you go to a casting call is though, dress for success. It's OK to wear just about anything, but have in mind that your look is your brand and could determine what role you get or if you get a part at all. Stick to the basics, the classics, and no sports clothing. You can dress casually, but dress to make a good impression. Also realize they are probably going to take your photo, even if you bring a professional headshot.