Search engine optimization used to mean attracting more traffic to your site by getting your site to rank higher on a page that displays search results for a specific query.
SEO by proxy is a new(ish) twist. Hijack a page on an already established site, optimize that instead for the query, and pepper in abundant links to your own original destination. Here's a document on Scribd that doesn't make any sense to anyone but a Google spider, which put it on the first page of results for "brand expectations," above a link to a much more legible but less keyword-stuffed book on Google's own Books site. The big red Click Here" links go to some affiliate page.