Open Call - an open call is a type of casting where the invitation to the casting is shared to a bigger mass of people, no just directly through an agency or casting site. These castings are usually open to anyone who wants to attend (within some guidelines.)
Pro-tip #1: There will be a million versions of people who may resemble you at open calls, be yourself. Don't be who you think they want, because you will just be forgettable.
Pro-tip #2: Get there ASAP. So many people will be there, so up your odds and show up early, to avoid the long waits and client fatigue.