I have this relay module. I don't understand what is a role of the jumper on it (for now it's shorting the jd-vcc (what??) and vcc pins).
The jd-vcc pin can be used to power the relay coils using a separate 5V power supply. Why would you want to do that? Each relay coil draws about 73mA @ 5VDC when energized. Theres a circuit schematic on this website: http://howtomechatronics.com/tutorials/arduino/control-high-voltage-devices-arduino-relay-tutorial/