DC stands for: Direct Current. DC voltage sources have a positive and a negative terminal. The symbol of a DC voltage source is
An example of a DC voltage source is a battery or a DC power supply.
To increase the voltage, you can connect multiple voltage sources in series:
The total voltage will be the sum of each voltage source. So when you connect two 1.5V batteries in series, you'll measure a total voltage of 3 volts.
By the way, most designers don't draw voltage sources in their schematics; they just draw the terminals:
The third drawing is the most common. The horizontal line at the bottom is the ground symbol. Ground is not always the negative terminal. Many audio devices for example use a so called symmetric power supply. Symmetric power supplies consist of two DC voltage sources connected to each other:
In these cases ground is the 'middle point' where both sources are connected. Ground is always the reference point. This means that all the voltages in the design or description are always with respect to ground. In other words: the black wire (connected to the COM bus) of your voltage meter should always be connected to ground, as shown in the picture above. In this case voltage meter M1 reads +9V and M2 reads -9V. And this immediately explains why it is called a symmetric power supply.
Later in this course you will learn what symmetric power supplies are used for and are we going to build one.