An intergreen period is the amount of time between the end of a green phase, and the start of another green phase.
For instance, this could be the time between the moment a 'green man' signal for pedestrians is extinguished, and the moment a green signal is shown for vehicular traffic. Alternatively it could be the time between a green light for motor traffic being extinguished, and a green light showing for motor traffic from another direction. The intergreen period would include the time the first signal goes to amber, any 'all red' period, and then the 'red and amber' shown before the green.