LSI keywords (Latent Semantic Indexing) are basically keywords that are semantically related to your primary keyword. Contrary to popular belief, they are NOT just synonym or keywords that are similar in meaning

LSI keywords are essentially keywords related to thekeywords that you are searching for, and Google does this by using a system known as LSI (Latent Semantic Indexing). In a nutshell, it is based on user search patterns and behavior, how one keyword search is usually linked to another keyword search.