A battery is a type of electrical device called a power source. Power sources provide electrical current that can be used in a circuit. Besides providing current to a circuit, batteries also store energy that can be used to push the current through a circuit.
A battery uses a chemical reaction to supply electric potential energy. Potential energy just means that the energy is stored for later use. Other types of power sources convert energy from one form into electric potential energy. For example, hydroelectric dams convert water pressure, wind turbines convert wind energy, and solar panels convert sunlight into electric potential energy.
Voltage is the measure of potential energy. To be more specific, voltage is a measure of the difference in electric potential energy between two points. For a battery, those two points are the battery terminals. A battery has two terminals, a positive terminal and a negative terminal. The terminals are marked on the battery with a + sign and a - sign.
The negative terminal of the battery is full of electrons (negative charges) that want to repel each other. The electrons are attracted by the positive side of the battery. So, when there is a conductive path between the negative and positive terminals, electrons flow out of the negative terminal and into the positive terminal.
However, there is a danger in directly connecting the positive and negative terminals of a battery. This is called a short circuit. Short circuits create a lot of heat because there are no components to transform the electricity into light, sound, or other forms of energy. Short circuits are harmful for the battery and could potentially burn you or even start a fire. When wiring a circuit, you should avoid creating short circuits. Always include a component that uses current such as an LED, resistor, or motor.
Electrons flow from negative to positive. But before electricity was fully understood, scientists such as Ben Franklin thought the positive charges moved through the circuit to create current. This is known as the conventional approach to current flow. In this approach, current flows from positive to negative.
This conventional approach is still used today by most of the world. All electronic components are manufactured with the conventional approach in mind. So, in a circuit, current flows from positive to negative, even though the electrons actually flow the other way.