In layman terms, it's so you don't stuff keywords like buy low carb wheat pasta online because low carb wheat pastas is the greatest pasta of wheat. Use latent terms to figure out related kw that will make for nice, clean and readable text - which will have a positive outcome for the reader.
In Simple words The search engine looks at similarity values it has calculated for every content word, and returns the documents that's call for LSI Indexing
The idea is that if a page has been written without SEO in mind it is likely that this page would talk about secondary themes, or keywords, related to the primary phrase. If you want to implement this throughout your SEO content the best place to start would be to look at Google, search for your primary phrase, then try to include as many of the related searches that it suggests into your content.