The black button is a test button and when pressed, should deactivate the outlet and any other outlet fed from it – Indicating a properly functioning device. The red button is the re-set button that you depress to reactivate the outlet or outlets in the event of deactivation resulting from a fault.
The TEST button is there to help you check and ensure the outlet is working properly. Whenever you plug a device in while the test button is on, the appliance should not work. The RESET button is there so you can return power to the outlet after running a test or if a breaker/circuit trip occurs.