Your first-stage regulator attaches to the cylinder valve and reduces the gas pressure from your cylinder to an intermediate working pressure of about 8 to 9 bar, or 116 to 130 psi.
Your second-stage regulator supplies you with breathing gas at ambient pressure (surrounding pressure) from the first-stage regulator.
You will often hear the terms “secondary regulator,” “regulator,” “second stage,” “octo,” “octopus,” “alternate air source,” and “demand valve” (DV) used interchangeably. These terms refer to the part of a regulator that delivers gas to your mouth.
Open circuit divers have two secondary regulators: the primary regulator and the alternate air source, sometimes known as the octopus, or octo. The octopus is for the unlikely event that your buddy is low on air or out of air and needs air. You will get lots of practice breathing on both the mouthpiece and octopus in your first confined water session.
For both OC and rebreather divers, the location of the octopus is in the front of the diver, between the chin and the end of the rib cage.