"LSI keyword" has become common SEO shorthand for terms and concepts closely related to your main topic, even though the phrase's technical origin is a bit different from how it's used day to day.
The Original Meaning
Latent Semantic Indexing is a decades-old information retrieval technique for identifying relationships between terms based on how often they appear together across documents.
How the SEO Industry Adopted the Term
"LSI keywords" came to mean, in practice, the related words and phrases that naturally accompany a topic — synonyms, subtopics, and associated concepts, not a literal output of the original LSI algorithm.
Why the Practical Idea Still Holds Up
Whatever the technical accuracy of the label, the underlying practice — covering a topic's related vocabulary instead of repeating one exact phrase — reflects how modern search engines actually evaluate relevance.
Next step: Use the LSI Tool to generate a list of related terms and concepts for the topic you're writing about.