Field Mappings are used to define the values assigned to fields for issues created by the Create Jira issues user action. They can be configured in the Jira Project Integration section of the User Management administration.
Field mappings define which project, issue type and reporter the issue should have and also define a number of "Field Entries". These can be used to set any supported Jira field type to a value of the user which the Create Jira issues action is running against.
For example, you could have the issues automatically assigned to the user they are based on or set a custom field to the user's email address.