LSI in simple terms is bringing together similar keywords and content.
Search Engine by the means of some similar search terms understands what a user is searching for.
Adding an example to this, the Search Engine knows when you are searching for an apple fruit and when you are searching for an Apple iPhone.
They know this because of the similar words used with it like when I am writing an article about apple juice – I will use terms like fruit, apple, doctor, mixer, etc.
On the other hand, when I am writing a blog on Apple IPhone I will use search terms like – camera, RAM, zoom, lens, storage space etc.
This reduces the concept of Keyword stuffing and improves the search engine’s ability to determine good quality content.