Electromotive Force
When a battery is connected to a light bulb in a circuit, current flows. The charge (electrons) experiences resistance as it goes through wire and light bulb.
This means that work has to be done against this resistance. The energy needed comes from the battery. If we want more current, or if there is more resistance for the same current, the battery must somehow be stronger.
The way to measure or specify the strength of a battery is called electromotive force.
Electromotive force is defined as the work done to move one unit of charge around the complete circuit once.
This is the exact definition, which is a bit abstract. What is one unit and what is charge? What is moving here are the electrons. In SI units, one unit would refer to 1 coulomb, or 1 C. In older days, or in some places, other systems of units are used, so it could be a different unit.
In SI units, the unit of work is joule (J). So electromotive force is the number of joules to move 1 C of charge round the circuit once. This is bit less now. For example, if 3 J of work is needed to move 1 C of charge round the circuit, we say that the electromotive force is 3 J per coulomb, or 3 J/C.
There is a unit defined for J/C, or joule per coulomb. It is called volt (V). So 3 J/C = 3 V.