LSI or Latent semantic indexing keywords are used to find the relationship or relevance between the main keywords and other keywords in the article.
For example, if your article is about "Inventory System" then the search engines find the keywords like "Inventory Management System" , "Inventory Management Software" , "Inventory Software" so that they can know the concept of the topic is to inform the user about the Inventory Management System.
Then the other keywords are called the LSI Keywords.
To check LSI Keywords, the best trick is to type in the Google Search bar and in suggestion you will get many keywords related to the topic you are searching for. Those keywords are LSI keywords.
For example, see in the image: