words are related in ways that aren't straight 'thesaurus' or 'dictionary' relationships. So 'coffee' has a relationship to 'milk' and 'sugar', for example, and in an article about 'coffee', you might also reasonably expect to find references to milk and sugar. In a similar way, an article about 'poker' may not actually have much keyword density for the word 'poker', but makes reference to 'Texas Holdem' and so on. This becomes useful when analyzing the text of the article to determine many things, such as 'was the article deliberately put together to try and game the SERPS', a subject dear to the hearts of most search engines. When creating an article, you need to ask yourself only one thing - does it look natural?
An LSI article is one with similar keywords to the main keyword including in the article. the old method was to stuff articles with the keyword 7-10 times in. The new way to write content is to include LSI keywords in the article.
Google is so in love with LSI, too. It's so important when you're trying to rank for certain (well, almost all) keywords. Do your diligent LSI research, it definitely pays off. Mike
If your website was about Windows, LSI helps the search engines determine the difference between "Windows software" or "glass windows"