LSI stands for Latent Semantic Indexing Keywords. One way of describing them would be the keywords that defines the targeted keywords. For example if you are targeting the search term "gain muscle" then typical LSI's would be "reps", "build muscle", "cardio", "weights", "protein". Hopefully you can see these are all the words (just some in this scenario) that Google associates with your primary keyword of "gain muscle". If you have all of the common LSI's in your content then Google knows you've covered the topic in detail and in general they will rank you higher in the SERPs because of this. They themselves have confirmed this. Semantically related words are an incredibly easy way to turbo charge your content.
Power LSI Finder is a desktop application that allows users to enter their niches keywords and Power LSI Finder will scrape Google in the background finding all the websites (up to 500 deep) that rank for that keyword. Once these websites (Google results) have been scraped it will crawl them all extracting their article content. Once its collected all this information it will aggregate all found LSIs from single 1 word LSIs to long tail 4 word LSIs. It not only does this for the content of all these websites it does it for the headers (h1 through to h6), keywords, title and description also. This is an extremely powerful keyword tool.
SEO has amazing ROI (return on investment), if you write a fantastic indepth article it can bring you traffic for years. However many SEO novices neglect to do indepth research and find all the possible LSIs for their primary keywords. If those LSIs are not present then Google can not be sure you've covered the topic indepth or at least to the same level as your competitors. This tool s allows you to find these gems and make use of them easily. You don't need to be technical at all you just enter your keyword and click start.