Predefined search query is a search that layers on top of the searches performed in the web part to limit their scope.
For example, setting the predefined search query to Department:"Marketing" and then searching for 'John' in the web part will return people named John in the Marketing department.
The predefined search query supports Keyword Query Language (KQL) and FAST Query Language (FQL) syntax. See User Guide for more information on KQL and FQL search queries.
The predefined search query does not include prefix matching by default. Prefix matching can be achieved by using the wildcard operator ("*").
E.g. return results including Jo, John, Johnson etc: