As you said, the open keyword allows you to override classes, when used in the class declaration. Accordingly, declaring a property as open, allows subclasses to override the property itself (e.g., redefine getter/setter). That keyword is required since in Kotlin everything is “final” by default, meaning that you can’t override it (something similar to C#, if you have experience with that).
Note that your class is implicitly declared as open since it is abstract, hence you cannot create an instance of that class directly.