LSI is a system used by Google and other major search engines (Update: read here, thanks for the Tweets, guys) The contents of a webpage are crawled by a search engine and the most common words and phrases are collated and identified as the keywords for the page.
LSI also known as latent symantic indexing which adds an important step to the document indexing process. In addition to recording which keywords a document contains, the method examines the document collection as a whole, to see which other documents contain some of those same words.