Keyword clustering is a practice in Keyword Research where topically related keywords are grouped together in order to create SEO content around the cluster and not a single keyword.
The idea behind keyword clustering is that pages optimized for a group of similar keywords (a cluster) can rank for a large number of keywords and signal to search engines about the degree to which the topic covered by the page.
Keyword clustering has become increasingly popular due to advancements in natural language processing (NLP) and semantic understanding by search engines.