What are sessions?
So sessions refers to the how many individual people came through to your listing page or how many visitors you had on a given day. The conversion rate is the number of sales divided by sessions you had on that day.
One assumption here is that Amazon filters out your traffic from any device you use to login to Seller Central. This is standard procedure. But if you visit your listings from another device you don't normally use however, then that would get tracked as a page visit.