Can anyone provide me with a definition of a National Park Interpretation Specialist?

What are his or her job characteristics?

A national park interpretive specialist is nothing more than a person who explains to visitors what they are looking at. That is what interpretation in this aspect means, providing a definition and/or explanation of either art or natural phenomena.
In the national park setting, you’d need to be very knowledgeable on the detailed aspects of the specific park in which you are interested in working at. This knowledge will be invaluable in an interview for said position.