Voltage seems like a really basic concept you either have it or you don't. Although really Voltage is a difference in potential. Think about this what if you there are two voltages in a given circuit. For example the power input is 5V which is then put through a 3.3V voltage Regulator. OK, so measuring from the 5V input to Ground is 5V and of course the 3.3V to Ground is 3.3V but what voltage will show when taking a reading between the 3.3V and the 5V. Did you think it was 0? What about 1.7V, that is the difference in potential between the two points.
Let's take to the next level. Many DC power supplies are have Dual outputs, of course this is handy when prototyping a circuit with multiple voltages. The cool feature is by connecting the wires to the right jacks it's possible (and common) to create a negative Voltage. Yes 2 Positive Voltages become one positive and one negative voltage. Since Voltage is a difference in potential it's a matter of defining where Ground is. The same exercise can be done with a couple of batteries.
- Positive and Negative Voltages - Negative Voltages are most commonly found in Audio