It is nothing else other than the word that we search for, and keyword stemming is bringing up the variants for the keyword by adding prefixes or suffixes to it. It is also done by stripping the suffixes or prefixes. It is a technique used by the search engines.
Adding a prefix to the word
Example:- Adding "en" to the word "large" makes it "enlarge".
Adding a suffix to the word
Example:- Adding "ful" to the word "sorrow" makes it "sorrowful".
Pluralizing the word
Example:- Adding "ies" to the word "baby" makes it "babies".
This process helps webmasters and search engine optimizer to generate more traffic by creating words that are similar to their base keyword. Words generated with the help of the base word or stem are treated as synonyms by the search engines. For this reason, keyword stemming helps a web page to achieve greater relevance for search terms without the use of spamming.