LSI keywords are essentially keywords related to the keywords 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.
Enforcing your targeted keywords with it’s related keywords (semantically related) you increase your changes on ranking higher.
These keywords can also be found at the bottom of Google’s SERP when you do the search