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. They are words that would commonly be associates in a the same article. For example , you cant talk about painting or write an article about painting without mentioning brushes or easels or canvases or colours. These words on their own have no meaning to painting but in the context of the article they are related to the keyword