A flash unit is ready to fire once the main storage capacitor has been charged to a voltage at which the tube can be expected to trigger reliably. This situation is usually communicated to the user by lighting a neon lamp or an LED, and to the camera by sending a signal along the sync. cable which causes the camera to adopt a shutter speed at which flash X-Sync is possible.
Thus a flash unit has a 'ready light', and a flash cable or connector (with certain exceptions) carries a 'ready signal'. Note that the ready light or signal is asserted when the flash unit can be triggered, but it does not imply that the flash is ready to give full output. If a flash unit has voltage regulation, you will hear interruptions in the whining noise from the inverter when the capacitor is fully charged. If the flash unit has no voltage regulation, the inverter will run continuously, and in this case, as a rough rule of thumb, you should wait about twice the time it takes for the ready light to come on to ensure that the capacitor is fully charged.