Hey...
LSI means laten semantic indexing....it helps the search engines to find out what a web page is all about simply you can say That makes it easier for search engines to determine the topic of your site. It basically means to you that you shouldn't focus on a single keyword when optimizing your web pages and when getting links.
The web pages on your web site should be related and focus mainly on a special topic while using different words that describe the topic. Use variations of your keyword and synonyms.