Google's Latent Semantic Indexing also known as LSI is a form of their algorithm that helps them decipher exactly what a web page is about. For example, if someone does a search with the term Apple in the long tail search term, how does Google understand whether or not they are talking about the apple in which you eat, or Apple, the computer? LSI enables them to do that. In other words, Google looks at the other words on the page, and make associations with those words. They have come to understand that when certain words appear with each other, it helps define exactly what the search is about. LSI enables for example, Google to determine if a page is about Windows, the operating system, or windows which are in your house. LSI is important to understand by web site owners who search engine optimize their pages. Google's search engine key word tool is free to use, and shows synonyms of key words, which are important to use within the content. This takes advantage of Google's LSI aspect of their algorithm.