There are many signature components of the Reggio Emilia approach. But if you want the main component, I would say it's "documentation". Reggio Emilia schools have an unstructured curriculum where children get to choose to do projects of their interests. These projects could take a few hours or even a few months to complete. These projects are rigorously documented with videos, photos, scrapbook, etc.
Below is an article that summarizes the specialties of Reggio Emilia preschools. I've also included a link on the information center on Reggio Emilia school.