When you press the orange button, the ram extends to cock the marker. When you release it, the ram retracts. Just like hmcl281 said.
It does tell you that the mechanical parts of the solenoid valve are working, and allows you to check for leaks in both extended and retracted positions.
It's also nice because it doesn't require any electricity; the button does the same job as the solenoid inside the solenoid valve.
The "button" is a standard feature of most of SMC's solenoid valves. In industrial settings it can save a lot of time if you can check an individual mechanical function of a complex pneumatic machine without having to run the entire machine.