Stemming is a process used by search engines to reduce words to their grammatical roots (or stems) so that they can deliver matches for derivatives in the results.
For example, searches for "stemming", "stemmed" or "stems" would be reduced to the root "stem" and all variations would appear in the results as a consequence.
Stripping words of their suffixes is nothing new and stemming has been used in computer science since the 60s.
Using variants of your keyword is fantastic for variety as stemming means that they will all be considered by the search engines. Check out this useful stemming tool which generates all the possible derivatives of a keyword root.