LSI stands for Latent Semantic Indexing. This is a way for search engines to gauge a page's relevance to it's target keywords. LSI works on the premise that words used in the same context have the same meaning, so the more you use synonyms or variations of your target keyword within an article, the more relevant the article is for that keyword. Hope that helps. Have a good day!
And this is a very important concept these days. You can't keyword stuff. but what you can do is use LSI keywords which means basically words and phrases related to your keyword to send additional signals to Google about what the page is about. The Google index is pretty smart. It isn't just looking for keywords but looking at the entire context of the page to determine how relevant it is to what keyword it is evaluating for.
To find LSI keywords go to G00gle keyword tool, type in your target/seed keyword, do not use any of the filters, click search, click the "keyword ideas" tab and there is a list of keywords that G00gle thinks are the most relevant to your seed keyword. When I write content I use the seed keyword in the title, body, description and then I Also use the top five relevant "LSI" (remember that list) keywords in the article as well. This will ensure that G00gle understands without a doubt what keyword you are targeting. Hope it helps