Definition - Visits of Product Listing Pages
"Number of visits to your Amazon product pages inside 24 hours. During one visit a customer can open your product pages several times, so that the number of page views in your reports is often bigger than the number of sessions."
Key Metric to Increase Sales
Next to the Conversion Rate, increasing the amount of sessions should be the main goal to increase the overall sales. Ways to increase Sessions are:
SEO / Keyword Research (free)
The basis to increase a product's organic visibility and thus sessions on Amazon is proper keyword research. How to best place your keywords in Seller Central and improve the keyword rankings is described here.
Amazon Marketing Services (AMS) / Sponsored Products (PPC)
Using Amazon based Advertisement is the second big lever to increase your products' visibility and thus sessions.
Finally, as SEO & PPC knowledge grows among the seller community, the final and increasingly more important lever is external marketing.
Various external sources can be used to lead customers directly to your products on Amazon, such as Facebooks ads, Affiliate marketing (eg. through YouTubers that are known in a relevant niche), special niche forums or blogs.