PageRank is a name that refers to Google’s numerical weighting of each website for the purpose of measuring its relative importance.
Using PageRank, Google assigns a score from 0-10 for each webpage on the Internet.
The PageRank of a particular page is roughly based upon the QUANTITY of inbound links as well as the PageRank of the pages providing the links, which could also be stated as the QUALITY of incoming links (if we consider that incoming links with greater PageRank reflect a higher quality).
There are other factors that determine PageRank, but Google does not divulge its secret sauce for bringing this about.
Here is how Google describes PageRank:
“PageRank relies on the uniquely democratic nature of the web by using its vast link structure as an indicator of an individual page’s value. In essence, Google interprets a link from page A to page B as a vote, by page A, for page B. But, Google looks at more than the sheer volume of votes, or links a page receives; it also analyzes the page that casts the vote. Votes cast by pages that are themselves ‘important’ weigh more heavily and help to make other pages ‘important.'”
From an SEO perspective, that means it’s good to get links from websites that have higher PageRank.
In the video about Driving Traffic with the SEO Triangle, that’s what is meant when it is stated that not all incoming links have the same value.