For any information system, capturing data is only half of the equation. Users will need to be able to retrieve the data at a later stage, usually through searching, and Jira is no different. While fields in Jira are responsible for capturing and displaying data, it is their corresponding searchers that provide the search functionality.
All fields that come with Jira have searchers associated with them by default, so you will be able to search issues according to their summary or assignee, without any further configuration. Some custom fields from third-party add-ons may have more than one searcher available. You can change the default searcher by editing the custom field.
In the Jira UI, a searcher is referred to as a search template.