What are custom fields in Jira?

Custom fields in Jira collect information that is not available in standard system fields. Custom fields are always optional in Jira. What this means for you is that you can create a new custom field without changing existing issues. The existing Jira issues will not receive a value for the newly created custom field, even if a default value is defined for the field.

Read how to create a custom field here: How to create a custom field