The Traffic Light custom field stores the number of working days and the traffic light color. For more info, please refer to the introduction on Traffic Light Custom Field.

There can be multiple Traffic Light custom fields created for each issue.

Adding Traffic Light Custom Field

Choose > Issues. Select Custom Fields under the Fields section
Click the Add Custom Field button. Select the All and search for Traffic Light
Select the Traffic Light field and click the Next button.
Configure the name and description accordingly
Click the Next button when you are ready
Associate the field to the issue view screens

As the Traffic Light custom field is not editable, you need to place it in the Issue View screen for it to be visible

For more details on how to manage Custom Fields in Jira, you can refer to Jira documentation pages below