A search command that generates events or reports from one or more indexes without transforming them. Use generating commands at the beginning of the search string by specifying a leading pipe character before the generating command.
In Splunk Web and the command-line interface (CLI), a search command is implied at the beginning of the search string. By specifying the leading pipe character first, the implicit search command is not added to the beginning of the search. When you use generating commands, do not specify search terms before the leading pipe character.
Examples of generating commands include
search (when used at the beginning of the pipeline),
For more information
In the Search Manual: