Internal PageRank is a score for each page based on how your own links point around your site. It works like the original PageRank: a page passes a share of its score along each followed link, the shares add up, and the process repeats until it settles. A damping factor of 0.85 keeps it stable and models a reader who sometimes stops and jumps elsewhere.
The point is that not all pages are equal in the eyes of a crawler, and your linking decides which. A page linked from many strong pages inherits their weight. A page reached only through the nav or footer, or buried several clicks deep, gets little, even if the content is your best.
Topkay computes this over a crawl of your site and shows it per page in the link graph. Use it to find valuable pages starved of internal links, and to see where equity leaks: links to dead pages, to noindex pages, or out through tracking parameter URLs.
To raise a page, add contextual links to it from relevant pages that already have weight. To stop a leak, fix or remove links into dead ends.