The Concept behind Latent Semantic Indexing is that each concept / word has words and phrases that supports it. thus by using a group of words together it is possible to derive the main concept/word from them.. for example

i like basketball,soccer,football,hockey,golf,baseball and also like horse riding,swimming and running.

thus a sentence may refer to sports without including the word sports within it.  this concept proves useful for writing content with low keyword density which is vital for good seo.

however this must not bed used to excess as some usage of keywords in articles is essential. After you've chosen your keywords I recommended you to use this ranking SEO tool to track your keywords position. It's very easy and simple. you'll find that related keywords are more promoted than not-related keyword.