Theory:

Potential difference:

To understand the concept of potential difference, we are going to consider the flow of water as an example. Water always flows from a region of high level to an area of low level. Similarly, electric charges also move from the region of high potential to low potential. An electric current can flow in the circuit only if there is a potential difference in the circuit.

The above picture shows the movement of water from a high level to a low level.

Positive and negative terminal of a battery

The cell shown in the picture has a positive terminal and a negative terminal. Their potential difference makes the electrons in the circuit move when the two ends are shorted.
The potential difference between any two points in a circuit is the amount of energy needed to move one electric charge unit from one point to the other.
Unit of potential difference:
• We have the unit $$ampere$$ ($$A$$) to measure electric current. Likewise, the unit to measure the potential difference is $$voltage$$ ($$V$$).
• The device used to measure the potential difference between the two ends is called a voltmeter.
• The electric charges move from the higher potential to the lower potential, just like the water flow.
Voltmeter
Instruments used to measure smaller volts, in the $$millivolt$$ or $$microvolt range$$, are designated as $$milli$$ voltmeters or $$micro$$ voltmeters.
$\begin{array}{l}1\phantom{\rule{0.147em}{0ex}}\mathit{millivolt}\left(\mathit{mV}\right)\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}{10}^{-3}\phantom{\rule{0.147em}{0ex}}\mathit{volt}\phantom{\rule{0.147em}{0ex}}=1/1000\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}0.001\phantom{\rule{0.147em}{0ex}}\mathit{volt}\left(v\right)\\ 1\phantom{\rule{0.147em}{0ex}}\mathit{millivolt}\left(\mathit{mV}\right)\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}{10}^{-6}\phantom{\rule{0.147em}{0ex}}\mathit{volt}\phantom{\rule{0.147em}{0ex}}=1/1000000\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}0.000001\phantom{\rule{0.147em}{0ex}}\mathit{volt}\left(v\right)\end{array}$
Symbols
Have you seen this picture anywhere?
You might have seen this in railway lines as well as in power generating stations. It denotes that the potential difference between the two terminals in the circuit is very high.
Reference:
https://en.wikipedia.org/wiki/File:Palakolli_waterfall.jpg
https://pixnio.com/objects/battery-of-one-and-a-half-volts
https://commons.wikimedia.org/wiki/File:Voltmeter_2.jpg
https://pixabay.com/vectors/signage-safety-health-warning-sign-32739/