It was a popular brand of shoes in the early 70's--almost as popular with counterculture types as were Birkenstocks. I believe Earth Shoes were from Sweden, and the company's name was Kalso (with a slash through the "o"), but I may be wrong. Earth Shoes had a "negative heel" and a broad toebox. Some people hated them, but they got me through my third year of med school, when I was sometimes on my feet for 36 hours at a stretch. Another company took over the name, but it's not the same. As far as I've ever known, the song simply refers to the shoes.

Aloha,
Mark