Stemming is a process which truncates words to their root forms in order to save index space and expand queries. For example, the words Search, Searching and Searched can be stored under the same root, Search-. Therefore, if Searching is queried, Coveo Enterprise Search (CES) also returns documents containing the words Search, Searching and Searched. The root under which each word is stored is determined by the stemming algorithm (for more information, refer to How to Modify the Index Settings, How to Modify the Advanced Configuration Parameters).
See also: What Is Stem Confusion?