The first step in electronics is understanding the concepts of current and voltage. There is a hydraulic analogy that helps to understand electricity, but here we will try to cover how it works on atomic and electron level.
After hooking up of all the connections to the RAMPS 1.4 board we will need to provide power to the board. The standard 5V output of the USB connector is only sufficient to power the Arduino and the LCD display, so we will need external power. The minium power that needs to be provided is 12V x 20A (240W). But just to be sure we will use a 12V x 30A (360W) power supply.