Hello. I haven't found any good reference materials pertinent to showing how to draw circuits in different ways. Please look at the attachment. There are three colored rectangles. The one is red - I haven't figured out what it means. It isn't even connected with the circuit? Why is this part of circuit important to this circuit at all? What does these red rectangles mean? Are they called terminals? Does the left one mean the input voltage (so that left one is connected with battery which isn't depicted on the drawing)? Does the right one mean the output voltage (so I can connect that output point with whatever element I want)? How to understand the red one? It doesn't seem to be the ground. Please try to explain it in plain english because I haven't taken any electronics course, just trying learn it by myself.