Prefixes are an S3 concept that represent the path to a virtual folder and can be used in Storage Manager when filtering objects in an effort to add them to rules. They are a way to help you visualize the concept of cloud storage, given that no actual folders truly exist, and allow you to search for objects by using common strings.
For example, setting a rule that applies to the /foo prefix would apply to all objects that start with /foo, such as /foo/bar, /foo/bar1/bar2, etc.