I'm looking at this Specialized Vita, or will be, when the bike shop has one in.

What are Zertz inserts and how do they work, please? I understand that they reputedly smooth the ride but I don't know how, or what they are made of. I'm looking for a bike that will be capable on local tarmacced lanes and roads but that can also do the very occasional short stretch of gravel or surfaced pathway. The choice seems to be between a Vita or a Giant Escape.

Thank you!