Enum is a type that provide pre-defined values that are contextual to the property they are connecting to. Properties which take enums typically have a type which begins with "e" as shown below for a configurator type.
For imageSource, there are only three possible values that are valid: media, none, and scene. By fitting an enum block into this space, it will populate the select box with those constrained values, as shown below.
Enums allow you to set values to these types of properties without having to look up what the valid values are.