Latent Semantic Indexing (LSI) is a sophisticated search engine system that uses natural language processing to analyze the relationships between terms and concepts on a webpage, providing more accurate and relevant search results. LSI keywords help search engines understand the context of a user's search query, allowing them to return results that match the user's intent. While LSI is crucial for improving SEO and categorizing content effectively, it should be used naturally to avoid keyword stuffing and provide valuable, user-friendly content.