If you are not familiar with extra variables, refer to Ansible - --extra-vars (variables) command line option.
Extra variables can be defined a various places in Tower, such as in a Job Template, in Inventory, or in a Credentials Store.
From the Templates view:
From a Project:
In this example, two extra variables have been defined, using the YAML format. This would be the equivelant of using the --extra-vars command line option.