It was originally based on the concept of ranking a scientific paper by the number of other papers citing it, and how important the linking papers were. This is called “impact factor” has been important in academic circles for years (see screencap at right of the home page for “The Journal of Research in Personality.”)
The impact factor (IF) of an academic journal is a measure reflecting the average number of citations to recent articles published in the journal. It is frequently used as a proxy for the relative importance of a journal within its field, with journals with higher impact factors deemed to be more important than those with lower ones […]
Here’s the text from the original 1998 paper by Brin and Page describing Google that shows its roots.
Academic citation literature has been applied to the web, largely by counting citations or backlinks to a given page. This gives some approximation of a page’s importance or quality. PageRank extends this idea by not counting links from all pages equally, and by normalizing by the number of links on a page.