An independent object (IsDependent is false) is an object that can exist on its own without being part of another object. For example, Jobs, Job Groups, etc.
A dependent object (IsDependent is true) is a part of another object. For example, a JobStep cannot exist independently of a Job, so the JobStep is dependent.
Dependent objects cannot be directly fetched from the server: they are only fetched as part of the object they belong to. They also cannot be saved or deleted independently: they are automatically saved or deleted when the owning object is saved or deleted.