Keyword Stemming, a concept often used in the field of linguistics, is also of great value in optimization of your web pages.  In a nutshell, keyword stemming in SEO is the addition of stems to a keyword that allows one to create multiple variations of that word or phrase without appearing spammy or keyword over saturation.  This usually involves pluralization and the use of prefixes and suffixes.  For example, stems for the keyword "investigate" could be investigator, investigation, investigates, pre-investigation, uninvestigatable, reinvestigate.

In terms of getting more traffic to your site, stemming allows you to both expand the densities of your main keywords while also keeping the content interesting to read while also giving more relevant weight to those keywords for search engines.
Google and most other search engines use stemming technology in its search algorithim.  As stated by Google: “...when appropriate, it (Google) will search not only for your search terms, but also for words that are similar to some or all of those terms. If you search for pet lemur dietary needs, Google will also search for pet lemur diet needs, and other related variations of your terms. Any variants of your terms that were searched for will be highlighted in the snippet of text accompanying each result."