The On/Off buttons on this pane do the same thing the Power On/Off buttons do on your hand held throttle. It turns the DCC Rail A/B voltage On/Off.
Where the DCC system supports a concept of an "Idle" state for DCC track power, and JMRI has been updated to support that feature, the pane will include an "Idle" button. Exactly what the "Idle" state means is DCC system-dependent. An example is shown in the image below.