PageRank (PR) is the score that Google attributes to a webpage based on both the quality and the quantity of backlinks to that page from other websites. The rank only concerns these links, rather than other SEO components.
The PageRank score will consider the authority of the websites that link to a webpage. For instance, a low traffic website that has a low PageRank will be regarded as less authoritative than a high traffic website with a high PageRank.
PageRank has not been visible to the public or to marketers since 2016, so instead SEO professionals have had to work on the types of backlinks they have and the relevance of the links in place, among other factors.