Compressor programming please help

[Tds123]

Ok so i was reading the rule book and the pressure switch is supposed to start working at 115 PSI and turn back on at 95. The rule books says the cRIO must be programmed to react to the GPIO port that is connected to the pressure switch... The question is... what is a GPIO port?????

This is also my program for the compressor please tell me why this dosnt work please! i have no clue why.

#include "Compressor.h"

Relay compressor;
DigitalInput compressor_switch;

compressor(5),
compressor_switch(1),

if(compressor_switch.Get() == 1) {
compressor.Set(Relay::kOn);
sprintf(m_compressor, "Comressors is Off;");
lcd->Printf(DriverStationLCD::kUser_Line2, 1, m_compressor);
lcd->UpdateLCD();
} else {
compressor.Set(Relay::kOff);
sprintf(m_compressor, "Comressors is On.");
lcd->Printf(DriverStationLCD::kUser_Line2, 1, m_compressor);
lcd->UpdateLCD();
}