Custom field is a test case's attribute used to provide meta data on this test case.
Example of a meta data:
Open a project and go to the Settings section.
Go to the Custom Fields section.
To configure custom fields mapping for the project, click the Create button.
Specify a Key for the custom field and select Custom field from the drop-down list, then click Submit.
More detailed description of Custom Fields can be fount in the test cases section