When you have a call for cooling from the thermostat the R terminal on the thermostat is connected internally to the terminal labeled G. The wire typically connected to the G terminal is green and it connects to the G terminal on the circuit board.  When this terminal receives 24 V input from the thermostat it energizes a relay on the circuit board starting the indoor fan. At the same time R is connected to terminal G it also connects to the terminal labeled Y on the thermostat. This terminal is typically connected to a Yellow or Blue wire depending on what brand of thermostat wire was used during installation. This wire will connect to a terminal on the circuit board labeled Y. This terminal is only a junction point on the circuit board. It does not connect to anything internally on the board. There will be one other connection on the circuit board labeled C. This terminal is connected back to the common terminal on the 24 V side of the transformer and is placed there for connecting the 24 V power to the outdoor unit. When the thermostat calls for cooling 24 V power will be available at Y and C on the circuit board. There will be a separate thermostat wire bundle with a wire running from each of these terminals to a contactor coil in the outdoor unit.