Electrical circuits all require a common electrical point of reference, and we call this point ground. This is why we see the push button and LED sharing a common connection on both the breadboard and schematic diagram (as a reminder, refer to Figure 2.10.
For the simple circuits presented throughout this book and when working with your Raspberry Pi's GPIO pins, it will be practical to consider the terms negative and ground as interchangeable. This is because the negative side of a power source will be our common point of electrical reference (and yes, GPIO pins are a source of power, which we will explore more in Chapter 6, Electronics 101 for the Software Engineer).
As mentioned previously in the Reading the push button schematic connection section, in step 4, we diagrammed the ground point using an arrow symbol. Our ground symbol (made out of line segments) is one common variation...