In real world, these are keywords that search engines know to be related to certain keywords (because of the frequency of them being used together and/or describing same notion). For example, VR and Oculus Rift have strong semantic connection, therefore have high latent semantic index in regards to each other.
What's really important is that LSI keywords - are the best type of keywords if you want to avoid keyword stuffing.
There are some useful tools to find LSI keywords for your list of keyword like LSI Graph or our Related Keywords report.