Your assumption is correct; an "Open Shoot" simply means that there is not a particular assigned subject. The word "open" in this context means not restricted.
An assigned subject in a photography course could be something general like nature or street photography or something specific like architectural details, abstract art, or the color red. Instead, with an Open Shoot, you can take photos of anything you like.
When I took a photography class in Vancouver, Canada, Open Shoot was used in this manner. Here is a camera club in Florida that defines it in the same way, too.