LSI stands for Latent Semantic Indexing. The keywords which are similar to the main keywords is called LSI keywords. These keywords are very helpful in SEO. For getting good ranking in SERP, you have to use these keywords along with main keywords.
Latent semantic indexing (LSI) is a system search engines use to analyze the other words people use surrounding a given topic. LSI keywords are words and phrases with a high degree of correlation to your target topic. Google's algorithm uses them to help determine content quality and relevance to the search term.
Latent semantic indexing (LSI) keywords are simply keywords related to the topic your page is about, and with Google's Panda update, they are looking for more of a topic match instead of a keyword match.