>Search was full of low-hanging fruit. TF-IDF was well-known. PageRank less so but when the nascent internet has so much signal-to-noise it’s often hard to go wrong.

PageRank actually originates from Economics[0] where Input–output model tells you that you need to care about balancing inputs and outputs in order to have an efficient economy. Speaking of internet search engines and ranking websites inputs would be links(backlinks) and outputs would be ranked web documents.

[0] https://en.wikipedia.org/wiki/Input%E2%80%93output_model