Need a help with adjustable DC power supply
The green terminal is true earth ground. It is connected to the ground lug on the ac power cord, which should ultimately be connected to a ground rod. The output of the power supply would seem to be isolated from ground, meaning that neither the positive or negative terminal is connected to ground.

For work with operational amplifiers you probably want a symmetrical bipolar power supply, which outputs both a positive and negative voltage having the same magnitude. Both of these voltages are with respect to a third common output. Such a supply would likely have four terminals where the fourth was the green earth ground.